The most common and most likely answer to this crossword clue is the 10 plus 5 letter word PHOTOGRAPH ALBUM
I've seen this crossword clue in The Times
P
H
O
T
O
G
R
A
P
H
A
L
B
U
M
More clues leading to the result PHOTOGRAPH ALBUM
Buddy explains!
The straight part
We know that "picture book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case picture book = photograph album.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.