I've been working with Windows Workflow Foundation for over a year - and was lucky enough to implement a project that ended up being the first world-wide enterprise system in production based on Windows Workflow.
At the time, I struggled to understand the concepts behind workflow before documentation was publicly available. Even now that the RTM is out there, the SDK documentation remains a reference text that necessarily stops short of a full exploration of the concepts.
Essential Windows Workflow Foundation by Dharma Shuka and Bob Schmidt carries on in the great tradition of texts like Don Box's Essential COM (the author of the foreword) and is simply a brilliantly concise discourse on the fundamentals of Windows Workflow. I am in awe of Dharma and Bob's ability to explain this technology which represents a paradigm shift in how we will architect our applications from here on out. The more I look, the more I see applications of the Windows Workflow Framework.
Get this book. Read it. You will not regret it.