I have noticed a book titled "Framework Design Guidelines" by Krzysztof Cwalina and Brad Abrams on my colleague's desk last week and asked if I could borrow it for a few days. Now that I've read it, I strongly recommend it to any .NET developer considering himself or herself senior. I actually plan to buy the book as it will make a great reference.
The book provides a comprehensive overview of best practices for design of reusable components, most of which should be familiar to a good developer working with .NET for some time. Even though the advice in the book is based on the experiences of .NET Framework implemnentation, which is targetting millions of people, it is perfectly well applicable for smaller frameworks. If you have enjoyed books like Code Complete by Steve McConnell or Jeffrey Richter's Applied .NET Framework, you will like this one too. Check this book out!