[R] R book needed

[Hua Li]

Hi there,

I'm looking for advice on a R book that's for somewhat advanced user. 

I've been using R for a while and can do the basic analysis with no problem. My problem is that for many already existing commands, such as gsub, textconnection, list, etc, I don't use them, simply because I don't know their existence! Can someone recommend a good book that I can refer to and can learn R systematically? 

Thanks a lot!

Hua