Many professors will ask the libraries to set aside copies of the books you will need for class. They are kept behind the circulation desks of McCabe Library, Cornell Science Library, and Underhill Music and Dance Library. Most reserve items circulate for 3 hours and overnight.
To see if the book you need is on reserve, search Tripod (tripod.swarthmore.edu) using the "course reserve" option. You can search by course name or by course number.