For starters, you might want to look at this article by John Cook. Also make sure that you read “The R Inferno”.
There are many good resources on the R homepage, but in particular, read “An Introduction to R” and “The R Language Definition”.
Some very closely related stackoverflow questions:
- books-for-learning-the-r-language.
- what-are-some-good-books-web-resources-and-projects-for-learning-r
- suggestions-on-way-resources-to-start-learning-statistical-language-r
My favorite book on the subject: “Software for Data Analysis: Programming with R”, by John Chambers, the creator of the S language.