...you should stick to physical books, because it takes longer to read e-books:

Nielson says that based on the tests, reading on the iPad is more difficult then a normal book, as the story took 6.2 percent longer to read, with the Kindle being even slower. Some suggest that the new or unfamiliar interface of the ereaders would easily account for the variance in results and that over time this would be obviated.

I wonder if there's any way to rank reading comprehension (besides giving a lame, junior-high-English-class-type test). The few times I've read an e-book, my speed was down, but I was paying extra attention to each page (in part because the new format made me concerned that my comprehension would decline).