37signals.com, the makers of popular collaboration and personal organization software, BaseCamp and BackPack have made their Getting Real book available for free online. You can still buy a PDF or paperback version if you so desire.
I've just recently started reading this, and am very impressed with the ideas Im finding in there. I have a feeling that the techniques and ideas described in the book will help me make the products we work on better.
Definitely recommended, if you're interested in developing a great web application in an agile development environment.