The writing of textbooks and making them freely available on the web is an idea whose time has arrived. Most college mathematics textbooks attempt to be all things to all people and, as a result, are much too big and expensive. This perhaps made some sense when these books were rather expensive to produce and distribute–but this time has passed.
A few years ago when I first posted a list of mathematics textbooks freely available on line, there existed only a handful of such books. Now there are many. The list here has grown and grown and is perhaps in serious need of some kind of organization into topics. There are also now many other sites at which there are links to on-line mathematics books and lecture notes. This site is far from comprehensive and I have considered abandoning it. Many people, however, still seem to find it useful, and so I shall continue to maintain it for a while.