Experienced developers interested in learning more about programming in R have a fantastic resource in John Cook's "R programming for those coming from other languages." Cook's guide assumes an understanding of general programming concepts - as the title says, it's meant for those coming from other languages - and the advantage, then, is a to-the-point tone and general conciseness that focuses on the information needed to become productive with R, without a lot of fluff.
R can be a little harder to get started with than some other languages due to some fundamental differences. According to Cook:
R is more than a programming language. It is an interactive environment for doing statistics. I find it more helpful to think of R as having a programming language than being a programming language.
But Cook's guide concisely spells out key differences and need-to-know tips, and splits it all up into a series of sections based on concepts - vectors, sequences, types, and so on - so you can easily review. Check out the full guide if you're interested in getting started with or learning more about programming in R.