Open Textbook Library
Many institutions of higher education in North America have been involved with the creation of Open Textbooks. The Open Textbook Library is a central home that lists each of these resources and who produced them. As a central home for Open Textbooks it is a great resource.
The only caveat is that their browse feature is not as well developed. For example they have many disciplines under their Social Sciences link, but they only list two in the sub-menu. So you will need to use their search engine or use the broader category to browse if they do no list your discipline under their Sub-Headings.
