If you're subject to program applications in collaboration with the Exchange 5.5 platform, and are a bit familiar with VBScript and Outlook, this book is a must. Simply because it provides loads of useful examples which will get you started at once.
This book does not dwell into the each technology around Exchange 5.5 and Outlook 2000, but instead it gives you an overview AND lots of examples of how you can make the most of your Outlook 2000 clients and Exchange 5.5 servers.
It tells you briefly about the Outlook 2000 object model, COM add-ins, CDO 1.21, Exchange agents and other relevant topics used in conjunction with the Exchange-server technology.
I have read a lot of these books, but this is simply the best in its area. The examples are not taken from any help-files, but - it seems - from real world applications (you can see that in the way they differ from the help-files).