The Exchange Server 2003 Technical Reference Guide
This has been popping up in everyone's blogs. I hate to be left out.
Microsoft has just published the Exchange 2003 Technical Reference Guide. If you want to know the technical nitty gritty of Exchange, this 480 page document is for you!
I only wish I had had this guide when I wrote my Exchange Server 2003 24seven book! It would have made finding some of these super technical details a lot easier.