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.
Fischer, L., Hilton, J., Robinson, T. J., & Wiley, D. A. (2015). A multi-institutional study of the impact of open textbook adoption on the learning outcomes of post-secondary students. Journal of Computing in Higher Education, 27(3), 159-172.
“In some educational settings, the cost of textbooks approaches or even exceeds the cost of tuition. Given limited resources, it is important to better understand the impacts of free open educational resources (OER) on student outcomes. Utilizing digital resources such as OER can substantially reduce costs for students. The purpose of this study was to analyze whether the adoption of no-cost open digital textbooks significantly predicted students’ completion of courses, class achievement, and enrollment intensity during and after semesters in which OER were used. This study utilized a quantitative quasi-experimental design with propensity-score matched groups to examine differences in outcomes between students that used OER and those who did not. The demographics of the initial sample of 16,727 included 4909 students in the treatment condition with a pool of 11,818 in the control condition. There were statistically significant differences between groups, with most favoring students utilizing OER.”
“This is by far the largest study of its kind conducted to date—nearly 5000 post-secondary students using OER and over 11,000 control students using commercial textbooks, distributed among ten institutions across the United States, enrolled in 15 different undergraduate courses. In three key measures of student success—course completion, final grade of C- or higher, course grade– students whose faculty chose OER generally performed as well or better than students whose faculty assigned commercial textbooks."