Although a small collection of books, these open books are authored by professionals and peer reviewed. They also align with standard scope and sequence requirements. The books are available online and as a PDF, although some of them can also be ordered in print (~$30) or obtained in the iTunes bookstore.
“OpenStax is a nonprofit based at Rice University, and it’s our mission to improve student access to education. Our first openly licensed college textbook was published in 2012, and our library since scaled to more than 20 books for college and AP courses used by hundreds of thousands of students. Our adaptive learning technology, designed to improve learning outcomes through personalized educational paths, is being piloted in college courses across the country. Through our partnerships with philanthropic foundations and our alliance with other educational resource companies, OpenStax is breaking down the most common barriers to learning and empowering students and instructors to succeed.”