The Open Textbook movement focuses on the creation and use of books which are openly licensed -- in short, free and editable. These textbooks are also openly licensed with a Creative Commons meaning that dependent on the license, they may be legally displayed/uploaded/hosted, distributed, modified, etc. without fee, permission, or conducting a fair use analysis. (See Creative Commons for more information on the various CC open licenses.)
Many of the collections will have links to the same books, but each collection has items unique to their collection.
Additional books that are not technically "open" are also listed. They may be "free online" but subject to copyright, or they may be low cost options.