The most common and most likely answer to this crossword clue is the 4 plus 2 plus 7 letter word JUST SO STORIES
I've seen this crossword clue in The Times
J
U
S
T
S
O
S
T
O
R
I
E
S
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 = just so stories.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.