/uː/ in moon and /ʊ/ in book. Roughly three to one in favour of moon, with no spelling clue — but a strong clue in the following consonant.
Vowel teams
oo TEAM_OO_01
Useful pattern · Medium reliability · Very common · Intermediate · Reading
/uː/ long oo: moon, food, soon
/ʊ/ short oo: book, look, good
Where Wiggler teaches it: Skill 10: Diphthongs & Variant Vowels
Child explanation
Oo says /uː/ in moon and /ʊ/ in book. If there is a k after it, it is nearly always the book sound.
The rule
Try /uː/ first. Before k, oo is nearly always the short /ʊ/ — book, look, took, cook, shook — which is the single most useful clue for this grapheme.
What is really going on
Both values descend from Middle English long /oː/, which raised to /uː/ and then shortened in a subset of words, particularly before /k/ and /d/. The -ook rime is essentially exceptionless for /ʊ/, which makes it a genuinely teachable sub-rule inside an otherwise variable grapheme.
Position in the word
Before a consonant — Before k, /ʊ/ with near-total consistency. For example: book, look, took, cook, hook.
Middle of a word — Elsewhere, usually /uː/. For example: moon, food, room, pool.
End of a word — /uː/. For example: too, zoo, shampoo.
When this rule works
/uː/ is the default first attempt.
/ʊ/ before k is reliable enough to teach as a rule of its own.
When this rule does not work
The word is blood or flood, where oo is /ʌ/.
The word is door or floor, where oo before r is /ɔː/.
Made-up words that follow the rule. A child who reads these has the pattern rather than the vocabulary.
bloon, shook, froop
Exceptions
oo as /ʌ/: blood, flood. Two words, shortened further than the rest.
oo before r: door, floor, poor. R-controlled: /ɔː/.
Contrast pairs
moon versus book — The two sounds.
food versus good — Identical rime, different sound — the counter-example to any -ood rule.
cool versus cook — The k clue, in one pair.
Decodable sentences
Look at the moon from the pool.
I took a good book to my room.
The cook put food on a wooden spoon.
Teacher note
Give the -ook clue explicitly — book, look, took, cook, shook, hook, brook — because it turns the largest chunk of the /ʊ/ group into a single rime a child can learn in one go. That leaves good, wood, foot, stood and wool as the short list to learn separately, which is manageable.