If you are a SAS and/or SPSS programmer and are interested in learning R, Robert A. Muenchen has written an excellent book just for you!  I cannot tell you how many pages of this book I've dog-eared, it's that chock-full of information.  Not only do you see side-by-side comparisons of SAS, SPSS and R code, Mr. Muenchen explains the basics well and then moves onto more complicated topics, such as factors and the apply functions.  Mr. Muenchen then moves onto graphics and statistics.  To order this book from Amazon, click on the link: R for SAS and SPSS Users (Statistics and Computing).