Raphael Ritz has posted a tutorial he wrote giving a guided tour of a fairly involved sample product for Plone. This is a far more involved product (and thus tutorial) than is seen in the Archetypes developer's guide or the Plone Book's chapter on customisation. It feels very similar to this "howto" from Plone.org on Archetypes development, and Sidnei's introductory article.
I've taken it upon myself to go through and attempt to edit and/or enhance the tutorial to the point where it could be taken as a text for the Zope/Plone/Python courses we're planning to teach in the Toronto area. Will likely need to get a license from Raphael to tear it apart and integrate it with the Zope and Python sources (at minimum to reformat it for a nicely presented text).