The Wall Street Journal of August 20,2010 carries a story about start up Inkling's introduction of "four full-length interactive college textbooks . . . designed specifically for Apple's iPad."  The texts, from McGraw-Hill (no releated info was found on their site about this), are best sellers in economics, psychology, marketing and biology. An introductory offer has chapters selling, beginning Monday, August 23, for $2.99 and the books for $69.99. Prices will go up to $3.99 a chapter and to $79.99 for whole books after the introductory period lapses.

Jakob Nielsen did a short study comparing reading speeds of iPad, Kindle, and the printed book:

The iPad measured at 6.2% lower reading speed than the printed book, whereas the Kindle measured at 10.7% slower than print. However, the difference between the two devices was not statistically significant because of the data's fairly high variability.

Thus, the only fair conclusion is that we can't say for sure which device offers the fastest reading speed. In any case, the difference would be so small that it wouldn't be a reason to buy one over the other.

But we can say that tablets still haven't beaten the printed book: the difference between Kindle and the book was significant at the p<.01 level, and the difference between iPad and the book was marginally significant at p=.06.