I posted this on the Guitar Techniques board (hopefully mods delete that one). I'm looking for an in-depth book on music theory. So far, I have found Harmony and Voice Leading. Anything else like that which isn't as expensive?
Get your hands on "Harmony In Concept And Practice" by Allen Forte if you can. It can offer you so many unseen things that you missed.

Check these out -

Tonal Harmony - Stefan Kostka and Dorothy Payne (intermediate-advanced+Expensive)
Jazz Theory Book - Mark Levine
Fundamentals of Music - William Duckworth - (intermediate + expensive)
I'd look at the books they're using at your local music school. Here in Serbia, the musical education is very thoroughly done, we have elementary music schools, high music school, and of course the Academy.

Why am I telling this? Well...if you're not an English speaker, or have a hard time understanding the excessive theory books written in English, or in your country B is called H (like in mine) and Bb is B, it's best to learn in your own language, and what better than the books real schooled musicians are learning from.

If you re completely new to theory, look for a book called "Elementary Music Theory" or similar. A similar book must exist in your local bookstore where they sell books for school, and it's a bible of kinds if you don't know any theory. Something like the Walter Piston book. But I'd rather learn from my local "Marko Tajcevic - Elementary Theory of Music" version, since we speak the same language. And a lot of musical terms are very different in Serbian. I rarely hear the term "Septachords" from U.S. people, just an example.

Also, there are certainly books on Harmony, Solfege(we learn that it high school), Counterpoint, Two/Three Voice leading...you just need to ask your educated musician friends what they used or use.

As for advanced books, there are many...try searching for guitar ebooks on soulseek network or somewhere...
I am Žile, I'm from Serbia, and I like guitars... and beans of course...never underestimate well cooked beans!

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Quote by YA89
Tonal Harmony - Stefan Kostka and Dorothy Payne (intermediate-advanced+Expensive)

We used this in my theory class at college. It's a pretty good book, but not that expensive unless you get the most recent edition. Older editions are available very cheaply through amazon.com. It does go over the basic concepts, but kind of breezes through them, so you might want to go through something more basic first.