Hi,
I would suggest before starting the Prep Guide, better read the Little SAS text-book, because that would give you a better experience , in understanding the SAS PREP Guide.The reason behind giving you this suggestion is, I have written my exam last month, and by reading this way, it really helped me to a quite large extent. If your target is to give minimum a month time , then the best way would be reading both LITTLE SAS and Prep guide simultaneously chapter-wise, else if you have a target more than a month, then reading first Little SAS and then Prep guide is also fine. (But the first way wud be much better, to save ur time)
Little SAS text-book to me is like "Holy book for SAS programmers", because it is always handy to check out for basic doubts and the best part is its examples.
Good Luck!