WordPress Development

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Digging into Wordpress, Chris Coyer and Jeff Starr. Not only an excellent book, it has a remarkable "lifetime subscription" model:
"When you buy this book, you will instantly get the most current version. But also, you are getting a lifetime subscription to all updated (PDF) copies of the book. Updates will automatically be emailed to you as they are released."
Smashing WordPress - Beyond the Blog Another excellent book looking at WordPress as a CMS (and more). The second edition is due out in April 2011.
Smashing WordPress Themes: Making WordPress Beautiful Annoyingly, the release date has been pushed back from January 23 to sometime in the future. And the digital version some time after that. But based on Hedengren's first book, this should be a must-read. Update: it's now available to buy for the Kindle.
Build Your Own Wicked WordPress Themes (SitePoint) by Allan Cole, Raena Jackson Armitage, Brandon R. Jones, Jeffrey Way