Hadley (ggplot2 author) also has a book on the package if you want to get a solid foundation: here
there's always the book by the author of ggplot: ggplot2: Elegant Graphics for Data Analysis. i haven't actually read it, but i'd imagine it's pretty thorough. there's going to be a new edition coming out, coinciding with a newer version of ggplot, iirc. there's also the official documentation here (looks like they just revamped the site).
but i learned it just by figuring out what i wanted to plot, and then using google/stackoverflow