To get a good overview over the .NET library, download Lutz Röder’s Reflector. We use it daily to browse assemblies (e.g. mscorlib.dll in C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\) with the visibility setting View > Options > Browser > Visibility set to Public Items only. The few namespaces worth becoming familiar with at first are
System.Drawing and the
System.Windows.Forms namespace. Framework Design Guidelines, a book by Krzysztof Cwalina and Brad Abrams from Microsoft, provides further insight into the main design decisions behind the .NET library. And finally there is the very Google friendly MSDN online documentation. Just enter a particular type or method name and feel lucky (e.g. IEnumerable).