This free, online textbook is found at HippoCampus, so the book is indexed by topic, rather than using a table of contents. This makes it a little harder to access than the normal textbook. Some of the textbook content is written text, while other parts are multi-media.
“HippoCampus is a project of the Monterey Institute for Technology and Education (MITE). The goal of HippoCampus is to provide high-quality, multimedia content on general education subjects to high school and college students free of charge. HippoCampus was designed as part of Open Education Resources (OER), a worldwide effort to improve access to quality education for everyone.”