There's always
WikiBooks, if you want restriction-free textbooks.. From their page:
We are developing free, open content textbooks, manuals and other texts. We currently have around 120 textbooks in various stages of development, of which every page is open to your help section or the study help desk. revision and addition. Look at the community section on the right below to learn about the wikibooks community, or browse the bookshelves down on the left. If you need help with Wikibooks, visit the
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