Little off the SharePoint topic but here is a link for a sample chapter for a Windows Workflow Foundation book titled Essential Windows Workflow Foundation. Looks like it is Addison Wesley Development Series which in my option puts out some very good books. Looks like the book is geared for Windows Workflow Foundation and not specific for the Office 2007 implementation of WinWF but I think you will pick some gems that will directly translate to the Office 2007 implementation.