diff --git a/config.json b/config.json
index c17d22f..b25d696 100644
--- a/config.json
+++ b/config.json
@@ -495,6 +495,14 @@
         "prerequisites": [],
         "difficulty": 6
       },
+      {
+        "slug": "book-store",
+        "name": "Book Store",
+        "uuid": "5cda9fbb-709f-44c0-bddf-5cf2652eff01",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 7
+      },
       {
         "slug": "satellite",
         "name": "Satellite",
diff --git a/exercises/practice/book-store/.busted b/exercises/practice/book-store/.busted
new file mode 100644
index 0000000..86b84e7
--- /dev/null
+++ b/exercises/practice/book-store/.busted
@@ -0,0 +1,5 @@
+return {
+  default = {
+    ROOT = { '.' }
+  }
+}
diff --git a/exercises/practice/book-store/.docs/instructions.md b/exercises/practice/book-store/.docs/instructions.md
new file mode 100644
index 0000000..54403f1
--- /dev/null
+++ b/exercises/practice/book-store/.docs/instructions.md
@@ -0,0 +1,61 @@
+# Instructions
+
+To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts on multiple book purchases.
+
+One copy of any of the five books costs $8.
+
+If, however, you buy two different books, you get a 5% discount on those two books.
+
+If you buy 3 different books, you get a 10% discount.
+
+If you buy 4 different books, you get a 20% discount.
+
+If you buy all 5, you get a 25% discount.
+
+Note that if you buy four books, of which 3 are different titles, you get a 10% discount on the 3 that form part of a set, but the fourth book still costs $8.
+
+Your mission is to write code to calculate the price of any conceivable shopping basket (containing only books of the same series), giving as big a discount as possible.
+
+For example, how much does this basket of books cost?
+
+- 2 copies of the first book
+- 2 copies of the second book
+- 2 copies of the third book
+- 1 copy of the fourth book
+- 1 copy of the fifth book
+
+One way of grouping these 8 books is:
+
+- 1 group of 5 (1st, 2nd,3rd, 4th, 5th)
+- 1 group of 3 (1st, 2nd, 3rd)
+
+This would give a total of:
+
+- 5 books at a 25% discount
+- 3 books at a 10% discount
+
+Resulting in:
+
+- 5 × (100% - 25%) × $8 = 5 × $6.00 = $30.00, plus
+- 3 × (100% - 10%) × $8 = 3 × $7.20 = $21.60
+
+Which equals $51.60.
+
+However, a different way to group these 8 books is:
+
+- 1 group of 4 books (1st, 2nd, 3rd, 4th)
+- 1 group of 4 books (1st, 2nd, 3rd, 5th)
+
+This would give a total of:
+
+- 4 books at a 20% discount
+- 4 books at a 20% discount
+
+Resulting in:
+
+- 4 × (100% - 20%) × $8 = 4 × $6.40 = $25.60, plus
+- 4 × (100% - 20%) × $8 = 4 × $6.40 = $25.60
+
+Which equals $51.20.
+
+And $51.20 is the price with the biggest discount.
diff --git a/exercises/practice/book-store/.meta/config.json b/exercises/practice/book-store/.meta/config.json
new file mode 100644
index 0000000..4e48588
--- /dev/null
+++ b/exercises/practice/book-store/.meta/config.json
@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "book_store.moon"
+    ],
+    "test": [
+      "book_store_spec.moon"
+    ],
+    "example": [
+      ".meta/example.moon"
+    ]
+  },
+  "blurb": "To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts of multiple-book purchases.",
+  "source": "Inspired by the harry potter kata from Cyber-Dojo.",
+  "source_url": "https://cyber-dojo.org"
+}
diff --git a/exercises/practice/book-store/.meta/example.moon b/exercises/practice/book-store/.meta/example.moon
new file mode 100644
index 0000000..92e548c
--- /dev/null
+++ b/exercises/practice/book-store/.meta/example.moon
@@ -0,0 +1,65 @@
+import fold from require 'moon'
+
+BOOK_PRICE = 800
+DISCOUNTED = {[0]: 1.0, 1.0, 0.95, 0.90, 0.80, 0.75}
+
+
+contains = (list, item) ->
+  for i, elem in ipairs list
+    return i if elem == item
+  nil
+
+
+filter = (list, pred) ->
+  fold {{}, table.unpack list}, (filtered, elem) ->
+    table.insert filtered, elem if pred elem
+    filtered
+
+
+group = (basket) ->
+  bundles = {{}}
+  for book in *basket
+    added = false
+    for bundle in *bundles
+      if not contains bundle, book
+        table.insert bundle, book
+        added = true
+        break
+    if not added
+      table.insert bundles, {book}
+
+  return bundles
+
+
+optimize = (bundles) ->
+  -- Two bundles of 4 are cheaper than a bundle of 5 plus a bundle of 3.
+  -- Look for a book in a 5-bundle that can be moved into a 3-bundle.
+
+  bundle5 = filter bundles, (bundle) -> #bundle == 5
+  return bundles if #bundle5 == 0
+
+  bundle3 = filter bundles, (bundle) -> #bundle == 3
+  return bundles if #bundle3 == 0
+
+  b5 = bundle5[1]
+  b3 = bundle3[1]
+
+  for book in *b5
+    idx = contains b3, book
+    if not idx
+      table.insert b3, book
+      table.remove b5, idx
+      break
+
+  return optimize bundles
+
+
+
+total_price = (basket) ->
+  bundles = optimize group basket
+
+  fold {0, table.unpack bundles}, (price, bundle) ->
+    price + #bundle * BOOK_PRICE * DISCOUNTED[#bundle]
+
+
+{ total: total_price }
diff --git a/exercises/practice/book-store/.meta/spec_generator.moon b/exercises/practice/book-store/.meta/spec_generator.moon
new file mode 100644
index 0000000..ffa8ca4
--- /dev/null
+++ b/exercises/practice/book-store/.meta/spec_generator.moon
@@ -0,0 +1,13 @@
+int_list = (list) -> "{#{table.concat list, ', '}}"
+
+{
+  module_name: 'BookStore',
+
+  generate_test: (case, level) ->
+    lines = {
+      "result = BookStore.#{case.property} #{int_list case.input.basket}",
+      "expected = #{case.expected}",
+      "assert.are.equal expected, result"
+    }
+    table.concat [indent line, level for line in *lines], '\n'
+}
diff --git a/exercises/practice/book-store/.meta/tests.toml b/exercises/practice/book-store/.meta/tests.toml
new file mode 100644
index 0000000..4b7ce98
--- /dev/null
+++ b/exercises/practice/book-store/.meta/tests.toml
@@ -0,0 +1,64 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[17146bd5-2e80-4557-ab4c-05632b6b0d01]
+description = "Only a single book"
+
+[cc2de9ac-ff2a-4efd-b7c7-bfe0f43271ce]
+description = "Two of the same book"
+
+[5a86eac0-45d2-46aa-bbf0-266b94393a1a]
+description = "Empty basket"
+
+[158bd19a-3db4-4468-ae85-e0638a688990]
+description = "Two different books"
+
+[f3833f6b-9332-4a1f-ad98-6c3f8e30e163]
+description = "Three different books"
+
+[1951a1db-2fb6-4cd1-a69a-f691b6dd30a2]
+description = "Four different books"
+
+[d70f6682-3019-4c3f-aede-83c6a8c647a3]
+description = "Five different books"
+
+[78cacb57-911a-45f1-be52-2a5bd428c634]
+description = "Two groups of four is cheaper than group of five plus group of three"
+
+[f808b5a4-e01f-4c0d-881f-f7b90d9739da]
+description = "Two groups of four is cheaper than groups of five and three"
+
+[fe96401c-5268-4be2-9d9e-19b76478007c]
+description = "Group of four plus group of two is cheaper than two groups of three"
+
+[68ea9b78-10ad-420e-a766-836a501d3633]
+description = "Two each of first four books and one copy each of rest"
+
+[c0a779d5-a40c-47ae-9828-a340e936b866]
+description = "Two copies of each book"
+
+[18fd86fe-08f1-4b68-969b-392b8af20513]
+description = "Three copies of first book and two each of remaining"
+
+[0b19a24d-e4cf-4ec8-9db2-8899a41af0da]
+description = "Three each of first two books and two each of remaining books"
+
+[bb376344-4fb2-49ab-ab85-e38d8354a58d]
+description = "Four groups of four are cheaper than two groups each of five and three"
+
+[5260ddde-2703-4915-b45a-e54dbbac4303]
+description = "Check that groups of four are created properly even when there are more groups of three than groups of five"
+
+[b0478278-c551-4747-b0fc-7e0be3158b1f]
+description = "One group of one and four is cheaper than one group of two and three"
+
+[cf868453-6484-4ae1-9dfc-f8ee85bbde01]
+description = "One group of one and two plus three groups of four is cheaper than one group of each size"
diff --git a/exercises/practice/book-store/book_store.moon b/exercises/practice/book-store/book_store.moon
new file mode 100644
index 0000000..5dfe72b
--- /dev/null
+++ b/exercises/practice/book-store/book_store.moon
@@ -0,0 +1,4 @@
+{
+  total: (basket) ->
+    error 'Implement me'
+}
diff --git a/exercises/practice/book-store/book_store_spec.moon b/exercises/practice/book-store/book_store_spec.moon
new file mode 100644
index 0000000..4fda14a
--- /dev/null
+++ b/exercises/practice/book-store/book_store_spec.moon
@@ -0,0 +1,92 @@
+BookStore = require 'book_store'
+
+describe 'book-store', ->
+  it 'Only a single book', ->
+    result = BookStore.total {1}
+    expected = 800
+    assert.are.equal expected, result
+
+  pending 'Two of the same book', ->
+    result = BookStore.total {2, 2}
+    expected = 1600
+    assert.are.equal expected, result
+
+  pending 'Empty basket', ->
+    result = BookStore.total {}
+    expected = 0
+    assert.are.equal expected, result
+
+  pending 'Two different books', ->
+    result = BookStore.total {1, 2}
+    expected = 1520
+    assert.are.equal expected, result
+
+  pending 'Three different books', ->
+    result = BookStore.total {1, 2, 3}
+    expected = 2160
+    assert.are.equal expected, result
+
+  pending 'Four different books', ->
+    result = BookStore.total {1, 2, 3, 4}
+    expected = 2560
+    assert.are.equal expected, result
+
+  pending 'Five different books', ->
+    result = BookStore.total {1, 2, 3, 4, 5}
+    expected = 3000
+    assert.are.equal expected, result
+
+  pending 'Two groups of four is cheaper than group of five plus group of three', ->
+    result = BookStore.total {1, 1, 2, 2, 3, 3, 4, 5}
+    expected = 5120
+    assert.are.equal expected, result
+
+  pending 'Two groups of four is cheaper than groups of five and three', ->
+    result = BookStore.total {1, 1, 2, 3, 4, 4, 5, 5}
+    expected = 5120
+    assert.are.equal expected, result
+
+  pending 'Group of four plus group of two is cheaper than two groups of three', ->
+    result = BookStore.total {1, 1, 2, 2, 3, 4}
+    expected = 4080
+    assert.are.equal expected, result
+
+  pending 'Two each of first four books and one copy each of rest', ->
+    result = BookStore.total {1, 1, 2, 2, 3, 3, 4, 4, 5}
+    expected = 5560
+    assert.are.equal expected, result
+
+  pending 'Two copies of each book', ->
+    result = BookStore.total {1, 1, 2, 2, 3, 3, 4, 4, 5, 5}
+    expected = 6000
+    assert.are.equal expected, result
+
+  pending 'Three copies of first book and two each of remaining', ->
+    result = BookStore.total {1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1}
+    expected = 6800
+    assert.are.equal expected, result
+
+  pending 'Three each of first two books and two each of remaining books', ->
+    result = BookStore.total {1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1, 2}
+    expected = 7520
+    assert.are.equal expected, result
+
+  pending 'Four groups of four are cheaper than two groups each of five and three', ->
+    result = BookStore.total {1, 1, 2, 2, 3, 3, 4, 5, 1, 1, 2, 2, 3, 3, 4, 5}
+    expected = 10240
+    assert.are.equal expected, result
+
+  pending 'Check that groups of four are created properly even when there are more groups of three than groups of five', ->
+    result = BookStore.total {1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5}
+    expected = 14560
+    assert.are.equal expected, result
+
+  pending 'One group of one and four is cheaper than one group of two and three', ->
+    result = BookStore.total {1, 1, 2, 3, 4}
+    expected = 3360
+    assert.are.equal expected, result
+
+  pending 'One group of one and two plus three groups of four is cheaper than one group of each size', ->
+    result = BookStore.total {1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5}
+    expected = 10000
+    assert.are.equal expected, result
diff --git a/exercises/test_helpers.md b/exercises/test_helpers.md
new file mode 100644
index 0000000..d739746
--- /dev/null
+++ b/exercises/test_helpers.md
@@ -0,0 +1,127 @@
+# Reusable stuff for spec generators
+
+## Helper functions
+
+Useful for generating pretty tables mostly.
+
+- List of ints
+    ```moonscript
+    int_list = (list) -> "{#{table.concat list, ', '}}"
+    ```
+
+    Used in: all-your-base
+
+- List of lists of ints
+    ```moonscript
+    int_lists = (lists, level) ->
+      if #lists == 1
+        "{#{int_list lists[1]}}"
+      else
+        rows = [indent int_list(row) .. ',', level + 1 for row in *lists]
+        table.insert rows, 1, '{'
+        table.insert rows, indent '}', level
+        table.concat rows, '\n'
+    ```
+    
+    Used in: saddle-points, spiral-matrix
+
+- List of strings
+    ```moonscript
+    list_of_words = (list) ->
+      "{#{table.concat [quote word for word in *list], ', '}}"
+    ```
+    The `quote` function is defined in bin/generate-spec
+
+    Used in: anagram
+
+- Indented multi-line list of strings
+    ```moonscript
+    instruction_list = (list, level) ->
+      if #list <= 2
+        "{#{table.concat [quote elem for elem in *list], ', '}}"
+      else
+        instrs = [indent quote(elem) .. ',', level + 1 for elem in *list]
+        table.insert instrs, 1, '{'
+        table.insert instrs, indent('}', level)
+        table.concat instrs, '\n'
+    ```
+    The `indent` function is defined in bin/generate-spec
+
+    This returns a multi-line string without the first line indented.
+    The returned string will be added to another string in the test case body,
+    and then that string (without regard to internal newlines) will later be indented.
+
+    Used in: anagram, twelve-days
+
+- Key-Value list
+
+    ```moonscript
+    kv_table = (tbl, level) ->
+      lines = {'{'}
+      for k, v in pairs tbl
+        key = if k\match('^%a%w*$') then k else "[#{quote k}]"
+        table.insert lines, indent "#{key}: #{v},", level + 1
+      table.insert lines, indent '}', level
+      table.concat lines, '\n'
+    ```
+
+    Used in: word-count
+
+- Show strings with escapes, when you want to keep the `\t`, `\n` in the test case.
+
+    ```moonscript
+    json = require 'dkjson'
+    -- and then
+      "result = Bob.hey #{json.encode case.input.heyBob}",
+    ```
+    ```moonscript
+    json = require 'dkjson'
+    json_string = (s) -> json.encode s
+    ```
+
+    Used in: bob, matrix
+
+- Table contains an element
+
+    ```moonscript
+    table_contains = (list, target) ->
+      for elem in *list
+        return true if elem == target
+      false
+    ````
+
+    Used in: forth
+
+- Wrapped list of ints
+
+    sieve has a test with loads of numbers.
+    This _should_ work fine on Mac or Linux ("Works For Me™)
+
+    ```moonscript
+    formatted = (list, level) ->
+      joined = table.concat list, ', '
+      cmd = "echo '#{joined}' | fold -s -w 76"
+      fh = io.popen cmd, 'r'
+      return "{#{joined}}" if not fh
+
+      lines = [indent(line\gsub('%s+$', ''), level + 1) for line in fh\lines!]
+
+      result = {fh\close!}
+      lines = {joined} if not result[1]
+
+      if #lines == 1
+        "{#{lines[1]\gsub('^%s+', '')}}"
+      else
+        table.insert lines, 1, "{"
+        table.insert lines, (indent "}", level)
+        table.concat lines, '\n'
+
+    ````
+
+
+## Custom assertions
+
+- dnd-character: `assert.between value, min, max`
+- space-age: `assert.approx_equal #{case.expected}, result`
+- word-count: `assert.has.same_kv table1, table2`:x
+