eXtreme Programming Explained
This book has profoundly influenced my views on enterprise software development. The book is told as a collection of best practices applied to a particular project and testifies to the project's success. The book emphasises software development as primarily a collaborative process, highly dependant on open communication between all members of the team. To me one of the most important concepts in the book is that all of the XP practices create a positive feedback loop, where change is welcomed and fully embraced into the process of software development.
The book is very easy to read and I found it profoundly insightful. It's one of my favorite books on software development and I would consider it a must-read for anyone involved in software development.