Good books on theory

well, the best books for practicing that aspect of theory is the real book (the legalized fake book). it's filled with old jazz tunes but to use it to its full potential, you should be able to site read. however this is a very hard book and you would have to work your way up to it. u also have to know all (and i mean all) of your chords. also, this book is for guitar
 
I just need something that encompasses shit like chord extensions, modes, circle of fifths, etc. Something I would learn in a basic music theory course
 
Harmony and Theory by Musicians Institute.

Trust me. It starts from the most basic and explains in layman's terms everything you need to know about basic theory and harmony. You can even download it on Soulseek or Emule if you want. My guitar teacher teaches at music academies and conservatories and he told me to get it to help with my instruction, and we go over it together.

I've got the Guitar Grimoire, but it's just basically a book of every scale and mode known to man without much instruction. I mean, there's a LITTLE at the beginning, but it's basically a practice book, not instruction.