Whatever you want to call it, MOUS or Microsoft Office Specialist, here are the books that will help you get certified. Many of these publishers also offer books on more than one topic. I might highlight their Powerpoint study guide here, but they may also offer Word or Access at the same quality. Be sure to do some additional searching based on this list for the best study guides for your needs.
1. PowerPoint 2000 Cheat Sheet
If you already have a fair amount of experience with Microsoft Office products, the cheat sheet series by Que will help you focus on what you need to know for the Microsoft Office Specialist exam.
2. Microsoft Office XP: Microsoft Office Specialist
Premier Press is on the ball and has published a series specifically for the newer designation of Microsoft Office Specialist. There are also books available from them for Word, Access, PowerPoint, and Excel 2000.3. Word 2000 MOUS Study Guide
This book is concise, focused, and very reasonably priced. It covers all of the information you will need to know for the Word 2000 exam.4. MOUS : Office XP
A new book by Gini Courter and Annette Marquis covering Office XP. With their names on it, it is bound to be an excellent book for getting the XP certification. This book covers all five XP exams.5. MOUS Essentials: Access 2000 with CD
Access is one of the more difficult exams for beginner users but this book should guide you through the study process with plenty of examples and a practice test on CD. The MOUS Essentials series also offers study guides for Word, Excel, and PowerPoint 2000.