From the post:
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. (Note that in most cases, resources are listed below by the course they are associated with.)
Covers a wide range of courses from computer science and engineering to languages and math.
I expect this resource will keep growing so remember to check back from time to time.