The researchers suggest that "the haptic and tactile feedback of a Kindle does not provide the same support for mental reconstruction of a story as a print pocket book does".That is from an article by Alison Flood at The Guardian shared in Marginal Revolution.
There is a minor point I would like to make, from a limited perspective. When comparing books and e-books, like Kindle-books, for example, we might be comparing different products. Let me tell what I mean. In several occasions I have bought Kindle books from rural areas in Guatemala. That means that from, lets say, Sipacapa, San Marcos, in the highlands (and that is far away from the capital) one can have in a matter of seconds a recent book from Amazon, for example, that might not be even available in bookstores in Guatemala City. Getting the same book in hard copy might take more time, may be weeks, and one might have to go to the customs office. So, basically, one can buy two or more e-books for the same amount of money that buys one book. Price differences probably exist as well even if one buys from a developed country. Frequently, having a Kindle e-book is, of course, better that not having the book at all.
If other things are equal probably books are better that e-books, but things are not equal in the "real world."