It could be anatomy, facial features, muscles, or skeletal system. Anything goes. So far, I've only finished reading "Fun with a Pencil" by Andrew Loomis(Thanks Mizu!) and I'd like to expand my knowledge more.
Although, if I have to be specific, I'd like to start off with the human anatomy. I'll be bookmarking all the recommendations here as well as go to my local bookstore to continue looking for art books. It doesn't matter if I have to pay for the book or not(I like free things though).
Thanks!