I get asked about this all the time, so here’s the best book I know of for learning music theory:
It’s fifteen dollars on Amazon.com. I’ve read a lot of books on theory, but Michael Miller is the only author with the balls to actually teach the reader the actual mechanics of music theory instead of just saying, “you should just memorize this stuff because I hate math.”
Trust me, this is how a lot of theory books are.
Buy it, read it, go through the exercises. If you find that you’re not finishing the book because you’re putting off doing the exercises, just skip them. Just make sure you get to the end and that you understand everything in it. If you know of any other cool music books, let me know!