My fellow guitarists. If you struggle with music theory, earned to play guitar using tablature, or play by ear, but want to get a better handle on music theory -- check out "Music Theory For Dummies." It came out in 2007. I read it recently and I think it is very accessible. My fundamentals were decent before reading it, but I still learned a few things. I wish this book was around early in my musical education. It would have made things easier.
[WARNING] The guitar neck diagram on page 89 incorrectly identifies 3 notes, but anybody who has played guitar for more than a couple of weeks will know it. Probably some idiot editor or printer is responsible.
On a side note, I would sacrifice virgins to the Circle of Fourths & Fifths out of gratitude and reverence...if I only could.