That is an excellent question.
While I do not have any direct answers, I do have some comments.
I think the market place for Biztalk related items is relatively small, thus you see a very small amount of resources available. For 2000 / 2002, a lot of the books ended up at the $5 books store. I’m not a publisher, but I’m guessing that is not a good thing.
Right now, the only book is the Biztalk 2004 Unleashed book. But, several places offer custom training classes and http://www.biztalk-training.com offers CD for sale.
On the plus side for 2004, Microsoft has put in a lot of effort to get free training and resources into your hands.
Some of the best is the MSDN Virtual Labs. In 2006, we will have a much improved SDK with full running Biztalk applications. I think that will help as well.
Plus, the Biztalk community is strong with a ton of great blogs and additional free resources available (like biztalkgurus.com :)).
Hope this helps.