I'm gonna be that person who recommends reading Marx because it provides a bit of a different answer than the ones already given. (Plus its the answer I'd give anyways I guess :P)

The Communist Manfiesto is a good introduction. There's Capital. A Critique of Political Economy, (Volume 1 in particular was completed while he was alive) which is a bit large, but its pretty good and still incredibly relevant. Socialism: Utopian and Scientific talks about a distinction made in a lot of Marx and Engels' writing.

These writings use the terms socialism and communism interchangeably (as opposed to some of the other answers here) because of a fundamental difference in perspective. It's one that is talked about in the first and third writing I linked, and somewhat indirectly in the second.