I don't know any specific e-books but there are plenty of decent websites if you google something like 'basic music theory'. For example,
this looks good.
As you progress, you might want to start thinking precisely what type of music theory you want to look into; for example the standard terminology, notation and theory of jazz diverges from classical after a while.
At first - you just want to learn the basics of music notation, the fundamentals of rythym, the diatonic scales, the basics of diatonic progressions and common voicings for diatonic chords and so forth.
Learning how to voice chords should be a big deal for most guitarists - it is one of the more versatile skills. You can have a lot of fun just taking a common progression and trying to find different ways to voice it using different inversions and octave displacements before even considering things like adding tensions, alterations and so on.
One thing that made me get into that is because I found that if I tried to play a chord progression just by shoving in the easiest (or only in some cases) grip for each chord - it generally sounded like total **** whereas by giving more consideration to voice leading and so forth, I found that with experimentation, I was able to get much better sounds. As an example, just take a ii-V7-I in Cmaj... if you play it using the seventh bar chords at frets 5 on the A string 3 on the E string and 3 on the A string - it will just sound like total un smooth ****e whereas, you could play something still simple like
e----------
B-5--4--3--
G-5--4--2--
D-3--3--2--
A-5-----3--
E----3-----
or
e----------
B-8---8---8--
G-10--10--9----
D-10--9---9---
A------------
E-10--9---8--
and you have descending voice or bass and it sounds smooth and properly outlines the progression.