These just keep getting better and better. I would love them for the in-depth history lessons about San Francisco alone, but add an engaging cast of characters, ongoing puzzles (there are a few unanswered questions spanning through several titles), and of course, the cats, and this is one of the few series I can say that I don't wait to read. I'll put down something else in favor of a new entry in this series.
In my opinion, this series goes a little deeper than most "cozy" series on the market. There's more history, and more loose ends, and the author has a very distinct "voice" that I really enjoy.
I can hardly wait for the next entry, How to Paint A Cat!