I purchased R in a nutshell for Oreilly and was pretty impressed with it, however a couple of glitches is worth mentioning. I'm reading it using my Sony PRS-600, but I also downloaded the pdf and mobi and tried those out on the PC. (The mobi version seems very similar to the EPub), here's my impression of the EPUB, snce I've spent most time with that one.
1. The font size looks slightly larger than the default-font on the PRS-600, the book is very readable on the small setting.
2. The epub is 3.7 Mb and it bundles two fonts (one of them monospaced) and many images.
3. All equations and graphs are images and high enough resolution to zoom in a bit on them. The equations was a bit oversized and one of the tables with equations was broken (The table called Distribution Funcion Families) all the graphs and figures looks great.
Despite the flaw with the table ... which really should have been fixed prior to publishing, the book works really well on my 6" reader. (I really don't want a larger reader, unless they can put a 7" screen in exactly the same form factor).
I'm seriously considering buying more epub's from oreilly ... the purchase is multiformat so you get epub, mobi and pdf in the same purchase.
And buying it using the 4cast code got me a 50% ebook discount.