+1 for Essential COM by Don Box, it still survives my bookshelf purges..."just-in-case". IUnknown and IDispatch are burned permanently in my memory from a period of my life building a COM/CORBA bridge.
I'm gonna piggyback on your comment to give another shout out to Essential COM. I haven't touched COM in ages but that book is so good I still pick it up every once in a while -- and it was a lifesaver back when I did touch COM on a daily basis.