Maria Konnikova at The New Yorker discusses the differences between reading online and reading on paper:
One of her main hypotheses is that the physical presence of a book—its heft, its feel, the weight and order of its pages—may have more than a purely emotional or nostalgic significance. People prefer physical books, not out of old-fashioned attachment but because the nature of the object itself has deeper repercussions for reading and comprehension. “Anecdotally, I’ve heard some say it’s like they haven’t read anything properly if they’ve read it on a Kindle. The reading has left more of an ephemeral experience,” she told me. Her hunch is that the physicality of a printed page may matter for those reading experiences when you need a firmer grounding in the material. The text you read on a Kindle or computer simply doesn’t have the same tangibility.