The Cluetrain Manifesto by Christopher Locke, Rick Levine, Doc Searls, David Weinberger
One of those books that make you go "Whoah !!" after reading it. I consider this book a must for anyone who considers the internet an important part of their lives or those who are considering making web applications development their living.
It doesn't really go into techie-mechie zone but rather delves in the realm of laymaness with easy to understand phrases that anyone who has at least used IE or mozilla to check their daily Yahoo mail will understand. A towering piece this one !