The most common and most likely answer to this crossword clue is the 12 letter word ENCYCLOPEDIA
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case book = encyclopedia.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ENCYCLOPEDIA mean?
ENCYCLOPEDIA noun- A comprehensive reference work (often spanning several printed volumes) with articles (usually arranged in alphabetical order, or sometimes arranged by category) on a range of subjects, sometimes general, sometimes limited to a particular field.
- The circle of arts and sciences; a comprehensive summary of knowledge, or of a branch of knowledge.