RC,
I have both books here in front of me, and several 600EX-RT's. I have highly recommended Syl's book for years, until recently it was a benchmark, but if you look at his sites you will learn very little extra from the book.
However, the newer Digital Guide book does carry an awful lot of detailed specific custom function and personal function advice for the 600 and the 580, the kind of stuff that isn't in the manual even if you can be bothered to find where it might be in there! It has more simple lighting diagrams and is cheap.
In a perfect world I would say for somebody starting out with a 600 EX-RT, get both, as I have both and the flash being talked about there is no doubt in my mind that the "better" book is currently the Digital Guide book.
But that is just personal opinion from somebody who has all three.........