Open textbooks are free, online learning materials with Creative Commons licenses. Many of the collections will have links to the same books, but each will have a particular focus, and items you can't find in other collections.
Multimedia Educational Resource for Learning and Online Teaching (MERLOT) is a curated resource of free and online textbooks for use in teaching and learning.
Initiated by Rice University, their free textbooks are developed and peer-reviewed to ensure they meet the scope and sequence requirements of college courses.