When designing class libraries quite a few things should be considered when laying your design out. Things like making your classes resilient, easy to understand, Intellisence friendly and extensible over time are just of few of the things but there are many more. Brad Abrams has a series of webcasts and a book that covers many of the things you should be thinking about when designing you class structure.
The webcast series Designing .NET Class Libraries was recorded at Microsoft and was originally presented to new developers at Microsoft. The series is a great way to see how Microsoft develops their APIs.
The book Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries is a continuation of the webcast series and covers topics ranging from the general philosophy of framework (class API) design to guidelines and examples of common framework design patters.