The most common and most likely answer to this crossword clue is the 8 letter word AGA-SAGA
I've seen this crossword clue in Financial Times
A
G
A
-
S
A
G
A
Buddy explains!
The straight part
We know that "Type of book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case type of book = aga-saga.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.