I finally passed this one after 3 attempts. By the time I was finished studying the 640 material I'd forgotten the material from 642 and vice versa. I bought the MS Press book for 648 2 or 3 years ago but never had time to go through it due to work. When I finally had time to read the book I found out it didn't cover the R2 material. I threw the book out and ended up using the MS press books (2nd ed.) for 640 and 642. I relied heavily on technet. Technet provides extra detail missing from the books. I also set up a lab to complete the exercises and installed roles and features multiple times. Now I'm on to 646.