OpenStax (openstax.org) is a nonprofit based at Rice University that publishes peer-reviewed, openly licensed college textbooks. They are free to read online, free to download as a PDF, and available in low-cost print if you prefer paper.
Coverage is strongest in exactly the courses most students take first: algebra, calculus, biology, chemistry, physics, psychology, sociology, economics, and U.S. history, among others.
The Student Hub at openstax.org/student-hub walks you through finding the specific book your class requires. If two of your four books turn out to be free, that changes your whole semester budget.
Sources: OpenStax, openstax.org and openstax.org/student-hub.