For the basics of reading harp music pretty much any book on the subject will do. 90% of what you see in harp music is nothing you won’t see in piano music.
The Kondonasis book already mentioned has a great glossary of harp specific notations and Salzedo effects.
Since you are a relative beginner you don’t need to focus on anything other than basic notation and perhaps pedal or lever notations. If you are learning on a lever harp then I recommend Sylvia Woods’ Teach Yourself To Play The Folk Harp. Even if you have a teacher it is a good book to have, and it teaches the basics of music reading along with teaching how to play. I was given a free copy when I bought a harp once and my teacher liked the exercises in the book and had me use some of them.
Also Woods’ book Music Theory and Arranging Techniques for Folk Harps is good once you have the basics down because it will teach you not just how to read, but interpret and make use of chord symbols and lead sheets.
From there you can worry about the more advanced notations that don’t often pop up in beginning and early intermediate arrangements for harp.