I'm a CS grad student at Caltech. My absolute favorite intro theory book (the one I recommend to undergrads and people just starting out) is Algorithms by Vazir

I'm a CS grad student at Caltech. My absolute favorite intro theory book (the one I recommend to undergrads and people just starting out) is Algorithms by Vazirani, Dasgupta, and Papadimitriou. Extremely well written and a great selection of topics.