Springer Mathematics Books
Springer, the academic publisher, has been maintaining an open access program for a while. They have released thousands of books in their website, free to download. Apparently it’s been going on for a while but I only found out recently.
Looking at the catalog, the selection is great. It includes hundreds of books from the Graduate Text in Mathematics series. In particular some of the books that I used in University. For example Lang’s Algebra
All the books in the open access catalog seem to be published on 2004 or before. I encourage you to browse through the catalog here.