It's a smaller book, weighing in at 1.5 Mb it has lots of images and code examples. The fonts used appear to be the same as in R in a Nutshell with excellent readability.
Browsing quickly through the first 100 pages of the book, looking for flaws. I noticed wrapping in a couple of places, but as far as I can tell doesn't interfer with the readability in those i discovered.
The book uses lots of images to illustrate syntax (reminds me of the Borland Turbo Pascal manual) and they are easily readable for the most part, but look even better when zoomed in slightly.
From my two samples, I'd say Oreilly is a good place to buy computer eBooks (Not really a surprise)
(A counter example is schaum's outline of discrete mathematics which i purchased from mobipocket.com, i just never got into it due to the poor production value)