The most common and most likely answer to this crossword clue is the 9 letter word DUODECIMO
I've seen this crossword clue in Financial Times
D
U
O
D
E
C
I
M
O
Buddy explains!

The straight part
We know that "a book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a book = duodecimo.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 DUODECIMO mean?
DUODECIMO noun- A size of paper, so called because it is originally made by folding and cutting a single sheet from a printing press into 12 leaves; (5 by 7¾ inches): 6.5 to 7.5 inches high, approximately 4.5 inches wide.
- A sheet or page of that size.
- A book having pages of that size.