The most common and most likely answer to this crossword clue is the 8 letter word TWELVEMO
I've seen this crossword clue in The Times
T
W
E
L
V
E
M
O
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 = twelvemo.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 TWELVEMO mean?
TWELVEMO noun- Duodecimo, or 12mo, a paper size, so called because it is cut 12 to a (huge, originally made) sheet
- A page, book etc. of that size