The most common and most likely answer to this crossword clue is the 1 plus 4 plus 7 letter word A FINE BALANCE
A
F
I
N
E
B
A
L
A
N
C
E
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 = a fine balance.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.