Free sample chapter: Understanding Exchange 2003 Data Storage
Sybex has posted a free sample chapter from my Microsoft Exchange Server 2003 24seven book. Chapter 4 is Understanding Exchange 2003 Data Storage. This chapter gives you information on storage usage, the database engine, restrictions, and looking at the internals of the database (of course, I did not get in to too much detail in this one.)
The chapter is in PDF format.