Patrick Schmid actually compiled together blog posts of Office 2007 UI Team Program Manager Jensen Harris into sections of topics about the new Ribbon UI.
As someone who do a lot of Office customization, I keep wonder sometimes why certain things are design in certain way inside Office 2007. This bible web site will more or less gives you the answer to the 4Ws of the Office UI.
Jensen also did a prezzo at Mix 08 about the Story of the Ribbon, you can download the .wmv and .pptx there. Its interesting to see some early prototype of Office 2007 like the photo below.