The MOUS is now just MOS (Microsoft Office Specialist).
"Microsoft Office XP, Microsoft Office Specialist", Microsoft. ISBN= 1-59200-046-0. $34.99 including CD with practice files. Microsoft publishes similar books for individual components of Office for $24.99 each so this is a real bargin since it includes the entire text of the 5 separate books. It includes Word, Excel, Access, Outlook, and Powerpoint.
I just got serious about sitting for the MOS exams and so I purchased a set of practice exams from MeasureUp and I'm glad I did. Even if you are quite comfortable with the material, the exams are likely to throw you because of their stupidity. I ran through a practice test for Access (the practice test tells you after each question what your score is so you know if you got the correct answer and give you the option to do over), and discovered just how stupid the testing harness is. For example, in one question it had me change a font, point size, and effect and told me to resize the control as necessary. So, I did what it asked and it took me 5 tries to figure out that if I didn't resize by using the "size to fit" menu item, it marked the answer wrong!!!!! You need to also be careful to save before pressing the next button. Sometimes it tells you to and sometimes it doesn't but you HAVE to or again, it marks your solution wrong.
The practice exams are pricey but if they can ensure a pass on the first try, they are worth it ($217).