http://www.guitarscience.net/index.htm
http://www.guitarscience.net/papers/guibook.pdf
Someone shared this on another forum a month ago.
It is a good (and also free) resource about how guitar and guitar electronics work from a scientific point of view (yes, there's math in there). I read some chapters and it is really interesting.
The theory section also includes some great info too: http://www.guitarscience.net/theory.htm