Another BizTalk 2006 book is now available on Amazon.com.
Pro BizTalk 2006 by George Dunphy and Ahmer Metwally is described as a high-end resource based on feedback from developers. It covers topics like scalability, administration & performance tuning.
I have not had a chance to take a look at this book yet, but it appears to cover some topics that are not covered well in the BizTalk help guide or in other books.
Amazon.com is also having a special so you can get this book along with the BizTalk 2006 Recipes book for one low price.