Domain-Driven Design by Eric Evans
I love this book because it offers the most elegant model for software design I've ever seen. All software is boiled down into four elements: ENTITIES, VALUE OBJECTS, SERVICES, and MODULES. Not only does it offer a great mental model for conceptualizing design, it has tons of real world examples. Chapter 5 alone is worth the price of the book. This book WILL make you a better designer and developer.
Check out my Architect Bookshelf compilation for more recommendations for software architects.