Everyone has books that they consider suitable for continued use as a reference. I have been selling or donating my VB6 books as I have been buying new .NET books.
Design Patterns
by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides - who are the notable Gang of Four (GoF). Admittedly, this book's examples are are in Smalltalk and C++, but all the C#/VB patterns books refer to this one, so it is a natural. Personal Rating: Very Good.
Design Patterns in C# by Steven John Metsker proved to be the logical .NET GoF follow-on. It adds to GoF, and ended up pretty much making me get that book too. Personal Rating: Very Good.
.NET Framework Standard Library Annotated Reference, Volume 1 by Brad Abrams, which covers the Base Class Library and Extended Numerics Library. This is not about patterns, but something more important - insight. You can get a taste of this book by reading Brad Abram's SLAR entries on his blog. Personal Rating: Excellent.
.NET Framework Standard Library Annotated Reference, Volume 2 by Brad Abrams and Tamara Abrams, which covers the Networking Library, Reflection Library, and XML Library. (I now have this book and it is in my to-be-read pile.)
Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries by Krzysztof Cwalina and Brad Abrams. Obviously there is too much by Brad here, but I guess that is the nature of his job. You could read the design guidelines on MSDN, but a book seems a better way to go. (I now have this one too.)