Heylą!
This issue is a little bit complicated.
I can tell you what I've studied for harmony, but these texts are really compositors-oriented, but I think are the ones that give you most, and you must study in these books a lot of classical stuff. If you are interested in a more pragmatical approach, well, there's a lot of books around and all them are ok.
The books I suggest (and where I've studied) are:
Walter Piston: "Harmony"
This book use a classical approach from beginning to the end, some peoples claim that exercises are not creativity-involved, but I think it's a good book to start from. It starts from essentials concepts to more advanced stuff. Anyway this is about "tonal" harmony, there's only a little addendum about dodecaphonics forms and atonal stuff.
Arnold Schoenberg: "Theory of Harmony"
This book it's more complicated and involves the changes that where made in the first years of last century, includes philosophical stuff about what dissonance is and must be, but I think it's the best book I've ever read.
Vincent Persichetti:"Twentieth-Century Harmony: Creative Aspects and Practice"
this is a cool book my friend! This is a must for every jazzist on the earth and you can learn a lot and I mean A LOT from it! It's a more advanced book, with excellent exercises and a lot of things to think about, like resonances, quartal harmony, polyharmony, polymodes etc.
This is really a must!
I suggest to study first Piston's book, then Persichetti and at last the good ol' Schoenberg.
If you're interested, there's also a good book from Pierce about acoustic, but this is more math-involved along with physics.
About websites there's
www.jazzitalia.net with some interesting things but this all in italian only
I hope this helps
All the best,
Arch
