Browse these sites to find free textbooks. See below for textbooks by a particular subject, major, or department.
What are Open Textbooks and why are they free? These books have been written by scholars (usually faculty at a university) who want their work to be freely used and/or adapted. Rather than going through the traditional publication process with high cost to the student, some of these authors are funded by charitable organizations so they can produce their work. Others do it for free, knowing the prohibitive cost of textbooks for college students. Many use Creative Commons (CC) licensing, which means you can potentially distribute, remix, adapt, and build upon the material in any medium or format as long as you give credit to the author. That's great for faculty and students!
Take a look at who the author is to get a better sense of where the information is coming from. For example: A Concise Introduction to Logic is a textbook written by Craig DeLancey, who is Professor of Philosophy and Chair of the Department of Philosophy at SUNY Oswego. This is hosted by the Milne Open Textbook platform which is "a catalog of open textbooks authored and peer-reviewed by SUNY faculty and staff."