TheServerSide.net has released another interesting Ebook for free: COM and .NET Interoperability.
This book covers al the aspects of working with COM components in .NET and .NET components in COM, by starting from an introduction/review of COM concepts such as IUnknown, IClassFactory, and IDispatch interfaces as well as Active Template Library and IDL and then moving on to cover the basics of .NET to COM interoperability before building on that with more advanced topics such as handling Variants and manually destroying a COM object.
An interesting download and a good reference if you have to work with COM and .NET (always an hard task in my experience! 
).