R programming for those coming from other languages
johndcook.com
johndcook.com
As others have mentioned, getting into the useful data structures and libraries is the next step here, because those are what makes R great. The book "R in a Nutshell" is a nice desktop reference, and the CRAN Task Views for your domain are a pretty helpful jumping off point: http://cran.r-project.org/web/views/
The R mailing list and Stack Overflow communities are also a tremendous resources.
The refcard is wonderful though. Personally, I found R bloggers really useful, just reading through all the articles is a nice way to learn the packages and tricks of the language (some of the code on the R help mailing list is also extremely useful here).
In terms of short, concise, wonderful books Data Manipulation with R is a favourite. Its only 150 pages, but it describes how to move your data from one form to another in the best and easiest ways. The SQL stuff and the regular expressions might already be known to the people here, but some of the material on dates, times and general reshaping of data was extremely useful (to me at least). R is not strong in this area, IMO and its nice to see some guidance in this area, as R packages often have funky data shape requirements.
One invaluable resource I forgot to point out was: http://www.statmethods.net/ Very concise, directed pages that help you understand common/acceptable ways to do specific things.
For plotting, I usually skip the base R plots and go with ggplot2.
This sounds good!
and some good explanation,
Also sounds good.
the book is essentially a commented version of the R introductory manuals.
The question that comes to mind: exactly how good are the comments?
Little bit of interesting history on this was just a google away: http://blog.revolutionanalytics.com/2008/12/use-equals-or-ar...
http://stackoverflow.com/questions/1741820/assignment-operat...
The = operator is only a "top level" construct, while <- can be used anywhere. Now exactly what those limitations are may be a bit subtle (the OP's point), so I also just use <-.