On my flight to Prague, I got the chance to read the book Professional WordPress Plugin Development by Brad Williams, Ozh Richard, and Justin Tadlock. After 9 hours of intense reading, all I can say is that I was amazed. This is one of the best resources I have seen to date for plugin developers who want to learn to do things the right way.

Professional WordPress is a book by Hal Stern, David Damstra, and Brad Williams that covers every aspect of WordPress.