If your required book is not an OpenStax title, two other libraries are worth searching before you spend anything.
Open Textbook Library (open.umn.edu/opentextbooks) is a searchable collection of openly licensed textbooks that have been reviewed by faculty at colleges and universities to assess quality.
LibreTexts (libretexts.org) is a large free library of college-level texts across STEM, humanities, and social sciences, organized by course rather than by title, which makes it easier to browse when you are not sure what you are looking for.
Both are free to read online and most titles can be downloaded. Check the license terms on any specific book if you plan to redistribute it.
Sources: Open Textbook Library, open.umn.edu/opentextbooks; LibreTexts, libretexts.org.