The R project webpage provides a succinct description of the R language and environment.

At the start, one might want to look at the official R manual An Introduction to R.
A good book to start learning is A Beginner's Guide to R.
Another gentle introduction to R is R for Beginners by Emmanuel Paradis.

The Use R! series of books by Springer is particularly helpful for R.

Introductory books and papers about R include:

Installation Binaries:
Reading material: