Boundless Textbooks (free, $19.99 for in-app upgrade) is an app designed to help college students save money on textbooks and study on the go. Boundless covers more than 20 subjects, including biology, business, chemistry, physics, and sociology. Introductory textbooks are free, while full-course books are $19.99 each. Boundless is something I could have used in college when I spent hundreds of dollars on books every year.
The book I checked out was a psychology textbook, Discovering Psychology: The Science of Mind. I always find it a little difficult to read things on my iPhone because of the small screen, but the words were easy to see and nicely formatted.