Openstax

Advantages: free, peer-reviewed textbooks for introductory university courses. Many file formats. New mobile app. Teacher resources and solutions manuals are also available for free. In a 2017 study of University of Georgia students, 86% of respondents found Openstax books to be of equal quality or higher compared to traditional textbooks (Watson, Domizi, Clouser, 2017).

Disadvantages: App is new and buggy. Selection of texts is slim. Account registration is required.