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...

Vectors are a very important mathematical concept and are often used in basic engineering science courses such as statics and dynamics. Paul Kirby is the creator of Vectorland, an online course that explains three dimensional vector analysis. The material presented on the site covers many topics, like dot and cross products, vector equations of lines and planes, shortest distance calculations, and...