It's incomprehensible to me that this hasn't been getting played up here, but Packt has released the Ubercart book infowonders announced a while back. I've picked up a few copies to pass out in the Commerce Guys Ubercart training at DrupalCon San Francisco and in my Drupal Commerce session. What I've seen so far is really great, and a lot of community modules and themes get some good coverage inside. I'm hesitant to recommend it, as I haven't given it a thorough read-through, but on a skim I noticed they even correctly instructed users on setting up Product Classes to save clicks on Attributes assignments. I'm guessing it's good.
The book has a specific website here: http://www.drupalubercartbook.com