Digital Textbooks for College?

doggkruse asks: "I recently purchased textbooks for the current semester in college only to end up with an empty wallet and a sore back. I have been looking into digital textbooks so that I will not have to lug around the real ones any more. I have found one site that seems to offer a very limited number of digital books. Does anyone know of a more complete solution. Especially one that works with Mac OS X. As laptops in classes are becoming more prolific, I think it is time to ditch the paper and save my wallet."