I was chatting to someone about Windows Workflow Foundation last night at the NxtGenUG meeting and I recommended a book but could only remember the author's name (and even then only one of them - poor Bob Schmidt, 2nd in line and up against a memorable name like Dharma Shukla!).
Anyway, it's an Addison-Wesley book entitled "Essential Windows Workflow Foundation" by Dharma Shukla and Bob Schmidt. You can find it at our friends Computer Manuals (the people who often have a stand at our events) here: Essential Windows Workflow Foundation.
It's very much about the fundamentals of WF, how it works and why it works that way. It's not a practical guide to designing applications with WF. It's a terrific book and I'd thoroughly recommend it for anyone that wants to understand more about Windows Workflow Foundation.