Online book to take your R (programming) skills to the next next level. The authors is quite influencial in the R community and "knows what he's talking about". This is for advanced R users!
One of the main attractions of using the R environment is the ease with which users can write their own programs and functions. The R programming syntax is extremely easy to learn, even for users with no previous programming experience. Once the basic R programming control structures are understood, users can use the R language as a powerful environment to perform complex custom analyses of almost any type of data.