Some GRC instructors feel very strongly against using a digital version of a textbook for their class. Please speak with your instructor before ordering a digital textbook, which cannot be returned if you decide you do not like the digital format, or you have to drop the class. Some classes use Cengage Brain (iChapters) for their classes.
Both Amazon Kindle and Barnes & Noble Nook provide some downloadable textbooks. They do not carry all publishers, however, so you may not find the textbook you need.