Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries (paperback)

Good as a lookup book when there's design discussions and you need more viewpoints and things to think about, but I wouldn't take every word as gospel. It states things quite black and white and sometimes I would even disagree 100%, or it would state with complete conviction what's right or wrong when it won't really matter and is more about team convention (example: states flat out that tabs are wrong and spaces are right - it won't matter for code quality or the generated output - while I do prefer spaces myself, it's more important you as a team have consistency, not to mention tabs has an advantage for many developers with a disability, so being so black and white is IMHO just wrong)