R can legitimately be called both a programming language and a statistical package. Many books address both the programming and statistical components of R, but invariably the discussion of statistical topics is more detailed than the discussion of programming capabilities. As a supplement, I've started the list of links below. Each of these sources deals specifically and almost exclusively with the the programming aspects of R: objects, arrays, loops and conditional statements, custom functions, debugging, and so on. I'll add to this list as I become aware of other sites.