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OASIS offers users worldwide the ability to search a range of OER materials including textbooks, courses and corresponding materials, interactive simulations, public domain books, audiobooks, modules, open access books, videos and podcasts on a variety of topics - from anthropology to zoology. OASIS also is the only tool that allows users to limit searches by creative commons licenses or by faculty review.

not direct link, but describes search engine

While many of these sites contain the same books, this list can be helpful. Choose the interface that works best for you.

Very comprehensive list