I was actually going to link to the gutenberg project! It was invaluable to me when I was in college. I was able to use it for a lot of papers I wrote as primary source material, without having to try to find the books!! I love that place.

http://librivox.org/ Is pretty cool too. It is an audio library of a lot of the books you will find on gutenburg. Its a volunteer organization, so many times the reader's voice with change throughout the book, but its awesome anyway. You can also volunteer to read for the project.