I really recommend O'Reilly's VI Pocket Reference. It sure helps with the learning curve and even now that I've been using vi for about (gasp!) nine years it still comes in handy when I need to remember an obscure command.
I don't have the pocket reference, but I bought "Learning the vi editor" published by O'Reilly.
Had to order it from the UK, couldn't get my hands on it in any book-store in my city and it was more expensive to get one from a bookstore in Lisbon
It also covers Vim and seems to be a very complete manual.
BTW, if you want to see Vim doing some *really* awesome things, join the #vectorlinux channel and pick kidd's brain