Literally almost everything I could get my hands on. I learned music theory back when i wa a guitar player. Andrew wassons tutorials on YouTube were helpful without being too boring. Meanwhile, subjects like counterpoint were almost impossible for me to learn on my own without the help of someone else. For these topics and almost any other topic out there the musicians talk forum on ultimate guitar are irreplaceable. The musicians there really know there stuff. Also the berklee harmony and music theory books were classics even if they were boring. Stuff from musicians institute.... Basically anything that berklee recommended I tried to get my hands on via PDF format.
