The best R books teaches you how to use it effectively. R is a language designed for statistical computing. Therefore it is very common among statisticians and economists. R is also a software environment with its own graphical interface.
As a language R is easy to learn but you also have to know how to use the graphics.
Best R books – overall
You can view R as a toolbox. The best R books describe exactly what you have in your toolbox and which tools to use in different situations. Since you can’t remember everything it should also be a good reference.
The most popular R book is The Art of R Programming: A Tour of Statistical Software Design, by Norman Matloff. It is a very good introduction that will take you a long way.
Best R books – graphics
R graphics is something that you can spend much time with. You can always improve and make your statistics prettier or clearer.
There are a couple of books on R graphics. One of the best is R Graphics Cookbook, by Winston Chang.
Last words of guidance
Learning R is a great way to take your statistics career to the next level. If you are willing to spend some time you can differentiate yourself on the job-market. It is also a great skill for working as a freelancer if that is what you want.