MIT OpenCourseWare has shared 45 free textbooks online. The books include a number of math and science texts, as well as a few foreign language titles.
Here is more from the site: “MIT OpenCourseWare shares the course materials from classes taught on the MIT campus. In most cases, this takes the form of course documents such as syllabi, lecture notes, assignments and exams. Occasionally, however, we come across textbooks we can share openly. This page provides an index of textbooks (and textbook-like course notes) that can be found throughout the OCW site.”
Follow this link to check out all of the books.