Here are the links which answers a commonly asked question, where do we go to read about design principles or design patterns.

OO Design

.NET Design Patterns

The SOLID Principles in C#


Exploring the Factory Design Pattern

Exploring the Observer Design Pattern

Implementing Singleton in C#,

Implementing Observer in .NET,

Implementing Model-View-Controller in ASP.NET

Enterprise Solution Patterns Using Microsoft .NET



 and recent one