Bookboon.com offers over 1,000 free textbooks online. Browse their collection here.
The Directory of Open Access Books (DOAB) includes academic, peer-reviewed open access books from a variety of publishers. To search the directory, check out their website here.
The Global Text Project publishes open electronic textbooks and makes them freely available online. Books covers a variety of topics, from Accounting and Algebra to Health and Information Technology. To search for a book, visit their website here.
The LibreTexts project is a collaborative partnership between multiple institutions to develop open access texts. The resources created by LibreTexts contributors are under constant revision students, faculty, and other experts. Subjects include sciences, social sciences, medicine, and the humanities. LibreTexts also offers Textbook Maps in an attempt to create open versions of existing texts. More information can be found on their website here.
These site was created as an index to the open electronic textbooks used in MIT OpenCourseWare courses. You can search by department on their website here.
OER Commons includes hundreds of free textbooks from higher education institutions around the world. To search their online catalog, click here.
Open Culture provides links to 200 free online textbooks from a variety of disciplines. To browse available titles and see what's new, click here.
OpenStax College offers free online textbooks that are meant to fit within the scope for most typical introductory courses across a number of disciplines. For more information, visit their website here.
Open SUNY Textbooks is an open access textbook initiative from the State University of New York libraries. The textbooks included on this site are high-quality and cost-effective. Visit their website here to browse the collection.
Hosted by the University of Minnesota's College of Education, the Open Textbook Library contains a number of free online textbooks covering nearly a dozen different subjects. All books included in the library have been reviewed by faculty from a variety of colleges and universities. See what the Open Textbook Library has to offer here.
Saylor Academy provides curriculum for its free courses online in downloadable textbook format. Check out their available titles here.
Textbook Revolution is a website run by students and dedicated to promoting the use of free education resources online. Books are available as PDFs and eBooks that can be read online. For additional information, check out the Textbook Revolution website here.