I would start with this site: http://www.biztalk24x7.com/v1/ There are a lot of good links to resources there.
Here are a few places that I would take a look at:
Understanding BizTalk Server 2006 http://www.microsoft.com/biztalk/techinfo/whitepapers/understanding.mspx
Virtual Labs http://www.biztalk24x7.com/v1/ContentTransformer.aspx?tag=0E7BC688-A30B-11DB-A534-F38155D89593