As I was researching more for my BizTalk mapping post
I came across very interesting posts (three part series) by Sandro Pereira (Always pleasure to read his work)
As of now only two of them has been published by him, so I will keep you updated whenever third part comes out.
How BizTalk Maps Work – Processing model (Part 1)
How BizTalk Maps Work – Deconstructing a map (Part 2)
How BizTalk Maps Work – The sequence of links (Part 3) (coming soon) :)
I went through the posts and trust me they are worth the time... so do yourself a favor and read it