Nowadays, I'd say the most definite resource on all details of the R language is Hadley Wickham's book.
Reading this, you'll get a very thorough understanding of how R works.
The book covers - among many other things - functional and object-oriented approaches to programming in R. Other chapters are devoted to the basic data structures and to performance issues in R.
Note that this really is a published, high-quality book that is freely accessible online.
