The most common and most likely answer to this crossword clue is the 9 letter word GAZETTEER
G
A
Z
E
T
T
E
E
R
Buddy explains!

The straight part
We know that "Reference book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reference book = gazetteer.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 GAZETTEER mean?
GAZETTEER noun- Journalist
- Publicist
- A geographic dictionary or encyclopedia, sometimes found as an index to an atlas.
- A newspaper.
- An alphabetical descriptive list of anything.
- To describe in a gazetteer.