I've rewritten the introduction, which is now online. It's now I hope a much better summary of the book and walkthrough for the grid. Thanks are due to John D and James P and many others for very helpful suggestions. I dont know how this open source thing has been for you, but I've found it essential to getting the book in shape. Even the knowledge that it will be exposed to the world immediately gives a kind of extra dimension to the editing.
new intro link
Almost there (now I just have to re-edit the rest of the book by next monday!)