The Library has selected "required" text at the service desk --- these are often referred to as "Textbook Reserve books or Course Reserves". The books can be checked out for 2 hours with your student ID. Don't forget that scanning some pages is possible sending the PDF to your email.
The Library also have more and more ebook course text. Check the library catalog.
If the Library does not have the textbook you need --
1. Check with your professor to inquire about the textbook to be in the library. The course instructor needs to request the book be placed on reserve in the library.
2. Where can you purchase textbooks?
Use the consumer sites, but do not forget to ask -- how are the items being ranked, who ranked them, is the site one I can trust.
The Library does not promote any specific site. The Library presents as information only so analyze the site you use.