R is one of my favorite tools and I heartily recommend it to any hacker for data visualization alone. I've used R to make sense of high-n high dimensional data and it's a delight (even if the syntax takes a little getting used to).

As Buckwild said, best way to learn R is by messing around with it; There's in-editor help and a REPL. Download the GUI for your OS, not just the command-line tool; it has nice package management and graphical output (to a quartz window in os x).

If you're thinking of using R for plotting as well as analysis, I heartily heartily recommend the ggplot2 package. ( http://had.co.nz/ggplot2/ ) -- there's a lively google group for support and the output is far better than comparable packages (like lattice, my second favorite).

The other famous book for R isn't actually for R -- it's for S. http://www.stats.ox.ac.uk/pub/MASS4/ R is the open-source superset implementation of S/S-plus.

Download R and give it a shot: http://cran.r-project.org/mirrors.html