There’s a wealth of information out there on VSTO and Office add-in development, but it can be hard to sift the wheat from the chaff. Here are the links I kept close at hand the last time I did Office development.
Docs for the Excel and Word interop namespaces:
Argument lists for built-in dialog box objects:
Excel function reference
There are a surprising number of things that can be done by using the right Excel function instead of writing a bunch of .NET code, and it almost always makes your life easier.
WPF / Silverlight Links
WPF and Silverlight aren’t VSTO-specific, but I used them on a regular basis to make custom views.
Silverlight 3 control reference: http://msdn.microsoft.com/en-us/library/system.windows.controls(VS.95,loband).aspx
WPF control reference: http://msdn.microsoft.com/en-us/library/system.windows.controls.aspx