guitarist getting into music theory

Hi everyone!!!

I'm an acoustic guitarist (still a fairly new one) hoping to learn electric guitar and eventually join a band. 

today I was talking to my friend (who makes her own music, is also emo, and is so super cool) about how she wishes she knew music theory and is now getting into it. she urged me to start NOW, that I was fairly new

i know the very basics (kinds of notes, names of symbols and names) from playing piano and being forced to do pages of level 1 theory. but i want to really really understand music and how to play it. 

can anyone tell me where i could start, what books to buy, yada yada?

--roZ