Understanding BizTalk 2006
BizTalk server (Technologies) is one of the hardest to introduce to developers for several reasons, most probably cause you don't get to see out come in early stages of learning it additionally the background of the learner has to be strong in deferent areas like (WCF/Web services, XML, XSD etc)
however this document is my fav when it comes to introducing it to anybody
http://download.microsoft.com/documents/australia/windowsserversystem/biztalk2006/Understanding_BTS06.pdf