The most common and most likely answer to this crossword clue is the 6 letter word HOW-TO
I've seen this crossword clue in The Guardian
H
O
W
-
T
O
Buddy explains!
The straight part
We know that "Instructive sort of book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instructive sort of book = how-to.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 HOW-TO mean?
HOW-TO noun- A brief informal description of how to accomplish a specific task.
- That gives advice or instruction on a particular topic