The most common and most likely answer to this crossword clue is the 6 letter word QUARTO
I've seen this crossword clue in Financial Times
Q
U
A
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T
O
More clues leading to the result QUARTO
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 = quarto.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 QUARTO mean?
QUARTO noun- A size of paper (7.5"-10" x 10"-12.5" or 190-254 x 254-312 mm). Formed by folding and cutting one of several standard sizes of paper (15"-20" x 20"-25" or 381-508 x 508-635 mm) twice to form 4 leaves (eight sides).
- A book size, corresponding to the paper size.