Rain
I've read earlier this week that some studies tend to show that our brain reacts differently to paper books - stuff about mental maps (freely translated here, the article was in French) and how other sense were involved.
I can totally buy into that.
A lot of things are missing when you are reading an image of a book.
No smell, for one thing. When you walk into Barnes and Noble, it smells like a bookstore. It may border on subliminal, but even one book has that certain smell.
And the texture of the paper. Paperbacks have a feel to them, and the heavier bond paper in most hardback books has a satisfying feel to it. I really like the heavy bond, glossy paper in some of my historical reference books, but these usually have photographs, so the glossy paper is warranted.
I personally enjoy the tactile presence of a real book.