Many informative books on COM have been released recently. Two books that give an excellent summary are Inside COM, by Dale Rogerson, published by Microsoft Press and Understanding ActiveX and OLE by David Chappell, also published by Microsoft Press.
In addition, COM is thoroughly documented in the Microsoft Developer Network Library. Some good sources for general descriptions of COM are:
Component Object Model Specification, Chapters 1 and 2 (MSDN Library, Specifications)
The Rules of the Component Object Model (MSDN Library, Technical Articles)
The Distributed Component Object Model: A Business Overview (MSDN Library, Backgrounders)