Definitive list of WordPress books
I've been plugging away with WordPress for some years now. In that time, I've never really taken time to fully understand the platform. It powers my site, and that's been enough. But I feel I need to know more about the system to get the most use out of it.
Please suggest any books that cover WP in some depth. I don't necessarily need WordPress 101, but I am particularly interested in making use of WP's extensibility, as well as building themes.