I learned music theory using the "classical" approach, which has helped me greatly since my knowledge is not limited to that from a guitar player's perspective. It's helped me understand how and why certain chords work w/o having to relate them to the guitar, which can be a bad thing sometimes cos it really limits you.
I used a book called "Rudiments And Theory Of Music" (the red book) by the Associated Board Of The Royal Schools Of Music (ABRSM), but I'm not sure if it's still available. You should be able to find one in a used book store though. I strongly recommend reading this book.