I've started reading "Drupal 5 Themes" by Ric Shreves, available at PACKT publishing. I've gotten through the first couple of chapters so far but have found it easy to read. So far, I haven't learned much I didn't know--but it has put into a single place lots of information that I would have gone to several sources in the past.

Once I've finished reading the whole book, I'll give my $0.02 worth on it, but so far it seems good. My only question at this point is...will there be an update for D6?