I had private guitar instruction as a teenager and learned music theory. At that time it was mostly in the area of jazz and jazz improvisation. Beyond that I am self taught. As have so many others, I have read Piston on Harmony and Fux for counterpoint as well as others. I read Tonal Harmony in Concept and Practice by Forte which was really helpful. But I just recently purchased from amazon for .01 Techniques and Materials of Tonal Music, With an Introduction to Twentieth-Century Techniques, by Benjamin, Horvit and Nelson. It is the most succinct and straight ahead treatment of the subject I have seen. And while there is no substitute for a formal education, for those of us for whom this is not possible, I would recommend taking a look at this book. It's almost an outline, distilling the essence of the subject which sometimes gets lost in other treatments. I first found it at openlibrary.org. I could not download it but I was able to electronically "borrow" it. When I started reading through it I knew I had to have it and found it on amazon for .01. It is still available on amazon, though I think the .01 copy is gone.
Yes, it is a fine book. It is designed to be very flexible - I use it for the Theory course I teach, and it is very adaptable. One can emphasize counterpoint, or not; the exercises encourage - or allow - for a number of different approaches. It really does help to have the companion book of short works for analysis, so as to see the principles in action. As a bound book, it is not inexpensive, but as the authors point out, it is really a text that can cover a two-year/four-semester sequence. So, you do get your money's worth!