Rough Type has an article on the first year long study of eReaders in schools.

As the study notes, one advantage of a book is that we make a “cognitive map” of a physical book as we read it:

When we read, we unconsciously note the physical location of information within a text and its spatial relationship to our location in the text as a whole … These mental images and representations do more than just help us recall where ideas are located in a given text. We use cognitive maps to retain and recall textual information more effectively, making them useful tools for students who are reading academic texts to satisfy specific goals.