Free online textbooks from Cambridge University Press. From the website:
Cambridge University Press is making higher education textbooks in HTML format free to access online during the coronavirus outbreak. Over 700 textbooks, published and currently available, on Cambridge Core are available regardless of whether textbooks were previously purchased. We recommend a Laptop/Desktop computer with Google Chrome for the best viewing experience. Textbook content is read only and cannot be downloaded. Free access is available until the end of May 2020.