What is the best book on guitar music theory?

Well of course it is. What's different about guitar is you play mostly chords. There are sixty two known ways to tune a guitar, as discovered by Hawaiian slack key artists, and fifty two of them have been used in recordings. If two Hawaiians play together, it is rather likely that they are tuned to two different keys. Everybody else uses "standard" tuning, and rock musicians are considered rebels because they use "Drop D" tuning. Martin Denny used to be fond of playing guitar chords on a piano, which certainly sounded strange. But music is music, and theory is the same for all.