WordPress Plugin Development Cookbook book is published by Packt Publishing and written by Yannick Lefebvre. Book contains 80 practical step-by-step problems with easy to follow solutions to most common problems developer can have when creating plugin for WordPress.
Headspace2 is great plugin for SEO. I prefer it much over other solutions like All-In-One SEO. But, problem with it is slow development and problems with WP3, unneeded features while missing features I need for my websites. So, I decided to work on my own version.