Boundless, a textbook service that lets you purchase and download the equivalents of what you were assigned for a class, has just released an update that adds support for iPad.
Most of the books on Boundless come in at around $19.99 and for those that have ever purchased a college textbook, you already know this is a fraction of what you pay for the physical hard copy. If you don't mind having to work a tiny bit harder to follow along with topics that may not be in the exact same order as the assigned textbooks, you can definitely save some cash doing so. Boundless also comes with study materials and supplementary tests and quizzes to help you work through different topics.
If you happen to try Boundless out, let us know what your experiences are. iPad support will no doubt make reading and interacting with study material even more convenient.