Allen Hatcher is a professor of mathematics at Cornell University. On his home page, he offers a link to download his book Algebraic Topology in its entirety. Published in 2002, the book is over 500 pages in length and is free to anyone wanting a personal copy. In addition to Algebraic Topology, there are three other books that are work-in-progress. Despite being incomplete, they could be useful to students in related classes. Several research papers written by Professor Hatcher are also given on this site.