The best architecture books are:
Inside SQL Server 2000 by Kalen Delaney
The Guru's Guide series by Ken Henderson. The last of the series is specific to internals.
SQL Server 2005 books are on their way. There is coverage of the SSIS engine in Professional SQL Server 2005 Integration Services by Brian Knight, Kathi Kellenberger, Haidong Ji, Andy Leonard, and a whole bunch of others.
K. Brian Kelley
@kbriankelley