BizTalk architecture whitepaper

BizTalk Server 2004: A Messaging Engine Overview is by far the best document in helping to understand the BizTalk.

Despite being written for BizTalk 2004 it is still valid for 2006, 2006R2 and 2009 and makes an especially good reading between taking a First Look at BizTalk 2006 and digging deeper into the details.