If you are planning to learn or already started learning Biztalk 2006, following two resources will be really helpful to you.
To get an overview on biztalk 2006, read this whitepaper first
Understanding Biztalk server 2006
and then start your journey with following guide,
Biztalk Blogger Guide
This guide has lots of information on advanced biztalk topics. It also has a section for begineers, where to start and various resourced for begineers. Thought this guide created during biztalk 2004 time frame, it has information on 2006 also.