Here are some books that I’ve found helpful
This book is great for getting the hang of data management with R. I found it really clear and helpful.
Written by the creator of ggplot2, this book presents how to make elegant plots in a thorough and clear way.
http://www.r-bloggers.com/ – Aggregated feed of R bloggers.
http://had.co.nz/ggplot2/ – ggplot2 Reference Manual by it’s creator.
http://en.wikibooks.org/wiki/R_Programming/ – Wikibooks R Programming section.
http://stackoverflow.com/questions/tagged/r – Stackoverflow is a great place to ask programming related questions or search for answers.
http://cran.r-project.org/doc/contrib/Short-refcard.pdf – R Commands Reference Card.
http://www.personality-project.net/r/r.commands.html – Quick summary of useful R commands.