When I was at TechEd Developer in Barcelona a few weeks ago, I sat in on a few Windows Workflow Foundation sessions, and the latest book on Workflow "Essential Windows Workflow Foundation" by Dharma Shukla and Bob Schmidt was being touted as the Workflow book.
Well, I finally found a few hours free to read the first few chapters, and can confirm - its excellent! If you want to know how it all works under the covers, which I think anyone who is serious about using workflow should know, then this is the book to read. It's not the sort of book you just pick up and glance through - its very precise and detailed. If you try skip reading a paragraph or two, you'll probably find you've missed a couple of key facts.
You can Download a Sample Chapter from the book, if you want to check it out yourself. Otherwise, go buy it.