Good books and lecture notes about category theory.

Categories for the Working mathematician by Mac Lane

Categories and Sheaves by Kashiwara and Schapira

And when you get bored of reading, let the Catsters take over. (78 videos on Category theory!)

Lang's Algebra contains a lot of introductory material on categories, which is really nice since it's done with constant motivation from algebra (e.g. coproducts are introduced right before the free product of groups is discussed).