The open textbook movement focuses on the creation of books are either free, or affordable/low-cost options. Many of the free textbooks are also openly licensed with a Creative Commons meaning that, dependent on the license, 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.