Not having read the O' Reilly book,
I can't draw a comparison between the two, but I have been extremely pleased with "R In Action" by Robert Kabacoff
and it can be found here:
It's a work in progress, in that some 90% of the book is written. Pre-ordering the electronic version gives you the ability to download chapters as they are written, plus a final e-copy (or hard copy if you pay more) when it's completed.
I have a high degree of familiarity with SPSS and SAS, and am learning R to get around the crazy licensing issues of the aforementioned programs. I have been very pleased with Kabacoff's book, as I had *no* familiarity with R before grabbing "R in Action." The publisher/author support a forum where purchasers can identify errors and/or make suggestions for improvements before the book goes to final press.
Not sure if it is competition for "R in a Nutshell" or simply an additional reference, but worth checking out if you want to learn R. It's been very helpful for me.