I was refreshing my mind about API design and wanted to see if there were any good resources online apart from the book that I have. Came across the following 3 hour lecture on how to design APIs which pretty much covers one of the better books I have read on API design. The presenter is the co-author of Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries which I found to be good and useful book.