[Fowler: Is Design Dead?] Fowler, Martin, Is Design Dead?, http://www.martinfowler.com/articles/designDead.html.
This is a web article that Martin Fowler continually updates. It is a must read for anyone who is interested in software design, especially in the context of agile development.
[Gamma et al 1995] Gamma, Erich, Helm, Richard, Johnson, Ralph, and Vlissides, John, Design Patterns, Addison Wesley Professional, 1995.
This is a software development classic and should be a standard reference for every software developer. There is also a great deal of knowledge available on the web about these and other patterns. I highly recommend a web search for the keywords design patterns.
[Fowler 2002] Fowler, Martin, Patterns of Enterprise Architecture, Addison-Wesley, 2002.
Another useful book of design patterns for use with the web, databases, servers, etc.
Design Patterns, Agile Development, and Simple Design
[Evans 2003] Evans, Eric, Domain-Driven Design, Addison-Wesley, 2003.
This is one of the best software design books in my opinion. It fits extremely well with agile development and an emphasis on simple design within a design vision as described in the Emphasis on Design chapter, though the terminology is different.
[Kerievsky 2004] Kerievsky, Joshua, Refactoring to Patterns, Addison-Wesley Professional, 2004.
An excellent book that deals with the hard-core coding problems of balancing the up-front use of design patterns versus having them emerge over time through refactoring.
[Martin 2002] Martin, Robert C., Agile Software Development, Principles, Patterns, and Practices, Prentice Hall, 2002.
This book is unfortunately misnamed. The value of this book is in understanding the drawbacks of various design patterns and in general how to employ design patterns in an agile development context. Highly recommended.
[Fowler 1999] Fowler, Martin, Refactoring: Improving the Design of Existing Code, Addison-Wesley, 1999.
The bible of refactoring. Enough said!
[Ambler and Jeffries 2002] Ambler, Scott W., Jeffries, Ron, Agile Modeling, Wiley, 2002.
This book contains a rich description of some lightweight ideas for documenting designs.
[Armitage 2004] Armitage, John, Are Agile Methods Good for Design?, IEEE Transactions, Jan/Feb 2004.
This is an excellent paper on user interface design for projects that employ agile development.