Here's the description from the webpage:
This technical reference guide presents a system architect's view of Exchange Server 2003. It includes a general overview of Exchange Server 2003 messaging system design, together with more specific details, such as services dependencies, Active Directory® directory service integration, Exchange System Manager architecture, routing architecture, SMTP transport architecture, X.400 architecture, Exchange store architecture, and cluster architecture. This information will help you design, maintain, and troubleshoot an Exchange organization and also develop custom solutions for administrators.
This detailed reference guide is not for beginning administrators and does not show you how to implement or maintain Exchange Server 2003. Instead, this guide is for Microsoft Certified System Engineers (MSCEs) and Exchange Server experts who want to take their knowledge about Exchange Server 2003 to the next level.
400+ pages of Exchange technical goodness packaged into a nicely formatted DOC. You want to read this document and you're sure to learn some stuff, no matter what your Exchange expertise!