A while back I finished reading Framework Design Guidelines by Krzysztof Cwalina and Brad Abrams. It's been ranked well on Amazon, and I can see why. Besides the general guidance one expects from a book like this, it had two other things that I really liked: (1) all the commentary from top .Net Architects, and (2) it provided a behind-the-scenes history of how the .Net Framework and Base Class Library came to be. Especially for practitioners who have made .Net their life (like myself), these are good things to see. I remember back when I started with .Net 1.0 in 2002, and how it's progressed from there to 1.1 and 2.0 and 3.0 and 3.5 and now "3.6" (i.e. .Net 3.5 SP1), so the book has been a good stroll down memory lane.
I'm impressed by the sheer volume of practical examples - this book is not some ivory tower theory pamphlet, but rather written from the first-hand experiences in the trenches.
I think the book got off to a strong start with "What makes a framework easy to experiment with?" As the brunt of it is a giant list of "do's" and "don'ts", it can get a little tedious at time, but it's still a very good read. I think reading an authoritative book like this also gives you a little more confidence that there's not something obvious that you're missing, as well as subtly increasing your code-smell.