There are lots of great articles available from Object Mentor. Here is a selection that I feel are essential for any developer. Caution: ahead there be PDF links!

First up, the “SOLID” design principles:

• SRP — The Single Responsibility Principle [PDF]
• OCP — The Open Closed Principle [PDF]
• LSP — The Liskov Substitution Principle [PDF]
• ISP — The Interface Segregation Principle [PDF]
• DIP — The Dependency Inversion Principle [PDF]

Then some general practices and patterns:

• Object Oriented Design Heuristics and Coffee – Robert C. Martin’s Coffee Maker example [PDF]
• The Bowling Game: An example of test-first pair programming – Robert C. Marin and Robert S. Koss
• The Humble Dialog Box – Michael Feathers’ classic article on MVC and testing GUIs [PDF]
• Working Effectively with Legacy Code – Michael Feathers on testing existing code [PDF]

At the very least I would read SRP, OCP and Coffee Maker if you go anywhere remotely close to Object Oriented Design in your work. Closely followed by The Humble Dialog Box for information on the MVC design pattern, and The Bowling Game to get an idea of what TDD is all about.