It depends on the classes. Most of my math/science books weren't at the library, so I had to buy them. My English books, however, were usually available at the library so I just checked those out for as long as I needed them.
I think that for English classes this might work if you are reading a specific novel. If it's a textbook though, good luck trying to find a library that will let you check out a book for more than 6 weeks.