For me it's not even a programming book... which is what I felt the people who have answered have artificially limited themselves to. For me, it's: http://www.

For me it's not even a programming book... which is what I felt the people who have answered have artificially limited themselves to. For me, it's: http://www.amazon.co.uk/dp/0195019199 http://www.amazon.co.uk/dp/0195019199 Pattern Language It's about architecture, buildings, towns. How to make them work, to serve all the needs of them, and how to allow them to grow. What is important to me and influenced me heavily is the thinking behind it. All parts of a large system in harmony, well-separated concerns, and working together to achieve a common goal. In architecture (computer as well as construction), there is also politics. We pour ourselves into these systems, our beliefs come out in their design and implementation. There was a lot that I learned from that book, and a lot that I still go back and refer to. GOF took their inspiration here, it's obvious from the structure... perhaps you should see why?