Getting Real (37 Signals)
First of all, this book is free. You can read it online here.
I love this book because it contains a quintessence of right/proper/correct/efficient approaches to web development in modern environment.
In brief, it is a collection of small articles about all sides of software development, that can turn your idea of web development upside down. Especially if you are a freelancer or work for a big company (not startup).
It’s really worth reading by everybody. Even if you do not develop web apps.