I would start with the Codex:
These two books are still current enough to remain great resources:
- Smashing WordPress Themes, Thord Daniel Hedengren
- Build Your Own Wicked WordPress Themes, Alan Cole, Jeffrey Way, et al
These also have Theme applicability:
- Digging Into WordPress, Chris Coyier, Jeff Starr
- Professional WordPress Plugin Development, Brad Williams et al
- Professional WordPress, Brad Williams et al
- WordPress Bible, Aaron Brazell
Most of the rest are either too general, or too outdated.
EDIT
Forgot to add Digging Into WordPress!
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