Not every book deserves the same decision. Running each one through a quick filter before you spend anything typically cuts a semester’s materials bill substantially.
Check free first. Search OpenStax, the Open Textbook Library, and LibreTexts by course subject before anything else. General education courses are especially well covered by free, peer-reviewed, openly licensed texts you can read online or download. Literature and philosophy courses that assign older works are often fully served by Project Gutenberg.
Borrow when the book is used lightly. If the syllabus assigns three chapters out of thirty, course reserves or interlibrary loan is enough. You do not need to own it.
Rent when the book is used heavily and the subject is outside your major. Rental is usually a fraction of purchase price and you were never going to open it again.
Buy used when the book is core to your major or has a workbook or access code. Anything you will reference in later coursework is worth owning. Check whether the access code can be bought separately from the book, since bundled codes are often the most expensive part.
Wait until the first class meeting when you can. Instructors frequently say on day one that an older edition is fine, or that a listed book will not actually be used. Course reserves cover you for that first week.
Sources: OpenStax, openstax.org; Open Textbook Library, open.umn.edu/opentextbooks; LibreTexts, libretexts.org; Project Gutenberg, gutenberg.org.