The most common and most likely answer to this crossword clue is the 8 letter word PERFECTS
I've seen this crossword clue in The Times
P
E
R
F
E
C
T
S
Buddy explains!

The straight part
We know that "Makes textbook" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case makes textbook = perfects.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 PERFECTS mean?
PERFECTS verb- To make perfect; to improve or hone.
- To take an action, usually the filing of a document in the correct venue, that secures a legal right.