Would you recommend one? If so which one? The stuff you see on the internets is sometimes hard to follow or just plain wrong.

Not on this site of course but...
I highly recommend both of these:

Harmony and Theory- imo, the best theory book I've come across. What makes it so great is that it's a workbook and pretty much the whole book, you have to stop and take your time to analyze and answer the work they give you each page. Forcing yourself to analyze and apply what they teach is a really great way to embed all that stuff into your head.

edit: this is the book they actually used in (and made by) the Musician's Institute, so if it's good enough to teach their students...

The Complete Idiot's Guide to Music Theory- Also, another great book. Teaches the same content as the above book, and a bit more, but in a much more casual way. Very easy to grasp what they teach and very good sections on how the theory is used in actual music (which Harmony and Theory doesn't go into much). Has exercises at the end of each chapter, but they are not as good, or as many, as in Harmony and Theory.

Best way to use both books (especially Harmony and Theory) is to do all the exercises. I'm a bit anal when it comes to keeping my books in good condition and intially only read through both books to avoid writing in them, but I felt I wasn't grasping the material as well as I should, so I said "f*ck it" and started filling out all the exercises.

The more I did it, the faster I got at thinking about all that stuff. When I'd go back to previous chapters to review, they would seem so simple because I could instantly answer them without much thought. It starts becoming 2nd nature and the less time and effort you spend thinking about those things, the easier they become to actually use on the guitar (or any instrument).

Best part is, you can get both shipped for about $25 total.
Last edited by alfaphlex at Jul 15, 2010,
Music Theory for dummies will get you off to a good start, it doesn't cover everything but there is some useful info in there and it's quite easy to understand - not so much musical parlance.