Reddit mentions of The Everything Music Theory Book: A Complete Guide to Taking Your Understanding of Music to the Next Level

A few points.

1.) learn the notes of the guitar
2.) learn some basic music theory
-learn about intervals (and their various shapes on the fretboard)

• learn what a major scale is
  
• learn about what a key is
  
  I think spengali is wrong about scales. Scales are very important. The first two scales you want to learn are the pentatonic scales and the major scale. The pentatonic scale is traditionally taught with five scales shapes (buy Fretboard Logic and learn about the CAGED system). The Major scale is traditionally either taught with 5 CAGED shapes or 7 three notes per string shapes.
  
  Let me give you an example. C major contains the notes (cdefgabc), if you want to create a solo in C major, that means that you play only those notes, and you mix them up creatively as you wish. Anywhere on the fretboard where those notes occur, you can play them. In order to make it easier to play them, the fretboard is broken up into sections. That's what the scale shapes are. In order to change keys, you just shift the shapes (if you're not sure what I mean, just ask).
  
  In the beginning it's best to just memorize the shapes without concern for what mode you're in. If you learn the 5 pentatonic shapes, you automatically learn minor pentatonic and major pentatonic. The shapes are exactly the same. The same goes for the major scale. If you learn the 5 caged shapes or the 7 three notes per string shapes, you've learned the major scale and all the diatonic modes too, including the minor scale (or aeolian as it's also called) because the shapes are exactly the same.
  
  When you play a solo, you essentially take the notes of a given scale and mix them up creatively. So you learn the scale shapes, learn how to connect the various shapes together, and you jumble the notes up creatively to make music.
  
  You might be wondering, if the notes of a C major scale and an A minor scale are exactly the same, and both are jumbled up creatively to make music, how can they be separate modes/scales?
  
  The answer is that it's the chord you're playing over the defines what mode you're in. The notes of a C major scale jumbled up over a C major chord will sound like C major and the notes of a C major scale jumbled up over an A minor chord will sound like A minor (or A aeolian).
  
  Edit: I'm in no way affiliated with this website, but the teacher is not only a great player, but very clear, and very thorough.
  
  http://www.guitarlessons365.com/
  
  Also, this is a good, easy to read book on music theory that won't make your head hurt.
  
  http://www.amazon.com/Everything-Music-Theory-Book-Understanding/dp/1593376529
  
  Edit 2: Just wanted to elaborate on my very incomplete and slightly misleading comment about modes. These links are a better explanation of them.
  
  http://www.guitarlessons365.com/scores/feb2010/UnderstandingModes.pdf
  
  http://www.guitarlessons365.com/scores/feb2010/WritingModalProgressions.pdf
  
  

Yeah, me before theory: "That's just a formalization of ideas I'm pretty sure I already know."

After learning some theory: "How the hell did I ever get by without this?"

Mark Schonbrun has two volumes in "The Everything..." series, here and here. They are not obviously guitar-centric from the covers, but they are more guitar/piano centric than anything else. The second book gets into voice leading, which really explains how to use chord inversions on guitar. (They are also relatively cheap.)

I think this has been posted here a few times, but it's a great book for introductory music theory.
The Everything Music Theory Book.