Domain-Driven Design: Tackling Complexity in the Heart of Software 1st Edition:

Eric Evans has written a fantastic book on how you can make the design of your software match your mental model of the problem domain you are addressing. "His book is very compatible with XP. It is not about drawing pictures of a domain; it is about how you think of it, the language you use to talk about it, and how you organize your software to reflect your improving understanding of it.