"Defensive Design for the Web: How To Improve Error Messages, Help, Forms, and Other Crisis Points" by 37signals (the minds behind this site) is now available. Guidelines, examples, and Head to Head comparisons make this book a must read for designers, programmers, copywriters, and any other site decision-makers who want to increase usability and customer satisfaction. If you find Design Not Found helpful, you'll really love the expanded content in the book. See for yourself.

"The folks at 37signals have created an invaluable resource: tons of 'best practice' examples for ensuring that Web users can recover gracefully when things — as they inevitably will — go 'worng.'"
-Steve Krug, Author, Don't Make Me Think