I would highly recommend getting the book Using Google App Engine by O'Reilly. It's a tutorial on Python, HTML, CSS and App Engine, and it's easy to skip the bits you already need. If you don't want to get the book, it's based on the content at http://appenginelearn.com/, which is all available for free.
Full disclosure: I was one of the technical reviewers for the book. I got paid 200 bucks for it, but I don't get any royalties - so there's nothing in it for me making sure it sells well. ;)