Drupal 7 Module Development has a couple of good chapters on developing modules that create and use entities, and there is another book out there on Drupal 7 Entities that I have yet to tackle.  I would also love to see something that shows how to implement "No SQL" high demand Databases like Mongo or Cassandra.  While I know I'm missing lot's on these newer flavors of database systems, I can't help but wonder if Drupal will ever run on a "No SQL" platform, or if that is where we are all headed. First glance suggests that perhaps Entity Classes will provide the keys to transitioning.