The Library has many of the books required for your classes. But we do not, as a rule, acquire textbooks. Confusing? We generally have classics in a field, which may indeed be your textbook. Also, professors often assign books that are not textbooks. Those, too, we often have. But the textbook market is vast, and textbooks are expensive and authors and editors update them constantly. Textbooks tend to address the fundamentals of a subject for a classroom audience. On the other hand, most of the resources of the library support individualized and specialized research in the disciplines. As a result, we may not have the particular textbook your course requires.
Still, it's always best to check. We may have an earlier edition or you may be able to request a copy from one of the other academic libraries in the DC area. Research Services can help you find the books you need, or you can simply search the library catalog Surveyor by title or author. For more info, check out this page.
And if your textbook is not available from the library, services like Amazon, Chegg.com, and Cheaptextbooks.com offer rentals and used copies. Additionally, select portions of some books can be read on Google Books free of charge.