The number of resources that you find for learning BizTalk Server is definitely less than those available for other MS Technologies ASP.NET or C# or Silver light.
Below is an approach which might be helpful for Beginners of BizTalk Server.
This approach assumes that you have good knowledge on C# and you are trying to Learn BizTalk Server from Basics.
1. Start by Reading the book Foundations of BizTalk server 2006.
I think, 2009 version has not come for this one. This is a very good book covering all the basic topics of BizTalk Server.
You can find the preview in the below link.
2. While reading the book in #1, practice the BizTalk Server Virtual Labs from Microsoft
3. The next one is to go through BizTalk 2006 Recipes: A Problem-Solution Approach
This one will have a quick and simples solution for various tools and shapes used in BizTalk Server Development.
4. Once the above books are done. Start reading advanced concepts on BizTalk. One of the good books available for this purpose is “Professional BizTalk Server 2006 R2”
Or you can also go through “Pro BizTalk 2009” or “SOA Patterns with BizTalk Server 2009: Implement SOA Strategies for BizTalk Server Solutions”.
I find that “Professional BizTalk Server 2006 R2” is rather simpler and easier to use that the other professional books.
5. The next step is to gain more experiences and in-depth knowledge in Mapping.
For this go through “Pro Mapping in BizTalk Server 2009”
6. For those who are working on BAM, there is an excellent one “Pro Business Activity Monitoring in BizTalk 2009”
Other useful Resources on BizTalk Server.
1. MSDN Blogs:
2. Video Tutorials
3. BizTalk Web Casts
4. BizTalk Server Documentation.
There are still lots of resources on this. I will keep updating this one when I find any thing useful.