While it is not really a "definitive" book, I would recommend "Switching to the Mac" by David Pogue as well. With 20+ years as a PC user under my belt (and many years as a enterprise network admin), I was not ready to read a "Mac for Dummies" type book. However, I found the Switching book by Pogue to be to-the-point for those that know PCs inside and out. It clearly laid out the key differences between the Mac and the PC. After a quick read of that, I was off and running on my new iMac. It certainly helped me.
I have the Leopard Missing Manual on order... 🙂
freddy