Exchange 2003 Administration Guide. Ok, so perhaps you don't need to buy an Exchange 2003 book right now. William told you yesterday about the updated Deployment Guide that's available, and once you've digested that, there's also the 487 page Administration Guide for your reading pleasure.
The book “shows you how to configure and manage Exchange Server 2003 organizations and servers. It also explains how to set up and use recipient policies, messaging, client access, mailbox and public folder stores, and Exchange clusters. It includes detailed appendixes on Exchange tools, services, store management, folder replication, and full-text indexing.”