The Berntsen Library usually does not have textbooks, but you can search for the book title in Library Search. Often textbooks have more than one edition, so you may want to check the publication year of the book you need.
If the library does not have the book, you can request it through Interlibrary Loan. Libraries, however, often are unable to borrow textbooks from other libraries.
Search Course Reserves, if your professor has put the textbook (or other materials) on Reserve in the library for your class to use.