Hadley Wickham is Mr. ggplot2 himself.  This book is an excellent and very detailed explanation of how to use ggplot2.  I highly recommend this book if only because I've nearly dog-eared every page!  A nice feature is that this book is in color and is not limited to a few color plates plopped in the middle of the book.  To order this book from Amazon, click on the link: ggplot2: Elegant Graphics for Data Analysis (Use R).