...and it's incredible.

http://www.amazon.com/Essential-Workflow-Foundation-Microsoft-Development/dp/0321399838/ref=pd_bbs_sr_2/104-2057410-5880735?ie=UTF8&s=books&qid=1183209452&sr=8-2

A few Amazon reviewers poo-poo it for being too esoteric and not dealing with "how to get work done in WF", or something.  This is definitely not a WF cookbook.  But the explanations behind bookmarking and continuations as the underpinnings of the WF architecture are fabulous.  Quite simply, if you don't understand this stuff, you don't understand WF.

The more I dig into this technology, the more fascinated I am.  The service model, the support for arbitrary execution semantics (but lack of preference for any baked-in ones), the multiple abstraction levels at which you can participate, the simplicity and yet power of the bookmark metaphor... it's really excellent stuff.

If you have even a casual interest in "next generation programming models" (as the tagline goes), read this book.