If you work with BizTalk or in any other way are involved with Business Process Management (BPM) systems, there is a fundamental read that I can highly recommend.
Martyn A. Ould's "Business Process Management: A Rigorous Approach" gives you all the essentials in the space of BPM. The theory behind BPM is explained rigorously and lots of practical examples and guidelines are presented.
The book examines BPM and approaches to identifying IT solutions involving BPM without reference to particular products or technologies (such as BizTalk).
If you've worked with BizTalk for a while and been occupied with the technical challenges of designing and implementing solutions, this book is an excellent opportunity to take a step back and get a good understanding of an area where BizTalk is being leveraged more and more.