I used Everything Music Theory. It worked well for me. It's well organized in a logical progression, and it explains how to use the information and why it is useful. I worked my way through it slowly, took notes, and made flash cards on Quizlet so I could make sure I really remembered and understood everything before I'd move on to the next chapter. You do need to know how to at least sort of read music though, so keep that in mind.
http://www.amazon.com/Everything-Music-Theory-Book-Understanding/dp/B007K4FNHE/ref=sr_1_2?ie=UTF8&qid=1373906333&sr=8-2&keywords=everything+music+theory
Also, musictheory.net is free and does a pretty good job at teaching the concepts, but the book I mentioned is much more in depth and easier to follow.