In the preface to the Data Visualization: A Practical Introduction author Kieran Healy writes:

My main goal is to introduce you to both the ideas and the methods of data visualization in a sensible, comprehensible, reproducible way. 

Well, mission accomplished. The book is at once enormously readable, and sufficiently technically detailed as to make it easy to implement the principles introduced.

The book itself is also beautifully designed. The use of figures and margin notes give you a sense of being guided through the ideas rather than just being told what they are. I've had lots of fun going back to some of my own visualizations made with R and ggplot2 and improving them based on what I learned here.

I absolutely recommend this to beginners and experts alike. Healy gives you everything you'd need to know if you're starting from scratch, but in such a way as to not slow things down for the more experienced reader. For that reason, it would also make a great book for a course on applied use of R.