This is definitely the best book on tonal harmony, simple and complete. Gives you all informations you need step by step. Sometimes it states "forget about this for now": well, it isn't really an issue because two or three pages after it explains what had left out before, but with more theory already explained at that stage. There're a lot of examples within the text and many tests to see if you've understood the theory.
You don't ever need a teacher to understand the concepts explained in this book: you really need one only if you're learning to play an instrument, not to understand chord functions, inversions, and cadences. This book teaches harmony, not piano or guitar solos.