Many guitar students get curious about music theory at some point during their studies. After all, a lot of us are 'thinkers' and like to understand how things work.
The guitar can seem like a mysterious instrument to a beginner, with little rhyme or reason... six strings (though sometimes seven!), a varying number of frets (depends on the kind of the guitar), multiple ways to play a single note, finger one chord. With just a little bit of background knowledge, though, things begin to make a lot more sense. A little music theory can go a long way on the guitar. Once we understand some fundamentals, we start to see patterns everywhere. Within weeks, we can find (albeit with some calculation) any note, and find a few different versions of a single chord. The more we play around, the more adept we become, and the better we understand what we're playing.
If you've been curious about music theory, I definitely recommend diving in. One book that I often recommend to students is called 'Edly's Music Theory for Practical People' It has a casual, conversational tone, yet explains the important concepts very clearly. Do all the exercises! I worked through about half of it before I applied to music school, and I ended up placing out of my first semester of theory - all due to that book!
Some students find Edly's a bit too cartoony (it IS cartoony, be warned!). If you prefer a more serious tone, I recently recommended 'The Practical Guide to Modern Music Theory for Guitarists' by Joseph Alexander to a student of mine. I haven't owned this book, but made the recommendation from looking at the table of contents, first few pages, and reviews online. So far that student is enjoying it.
What other theory resources do students and teachers recommend for guitarists? What theory knowledge has helped you the most to understand the instrument? Any books or concepts that you wouldn't recommend? How has acquiring some understanding of music theory changed the way you view the instrument? Any revelations or paradigm shifts once you understood certain concepts?