An Overview of ActiveX from Microsoft (100 Kb zipped Powerpoint 97 Presentation) - This presentation provides an introduction to ActiveX, and explains its relationship with COM and other Microsoft technologies.
"Building Your Human Machine Interface (HMI) using Visual Basic Technologies" - This 1999 IMS Show presentation by Software Toolbox founder John Weber includes an introduction to object terminology that is oriented towards the automation and control systems professional.
Chappell, David. Understanding ActiveX and OLE. Redmond, WA: Microsoft Press, 1996. ISBN 1-572-31216-5. This book answered a lot of questions for us about ActiveX and OLE, including many of the "I wonder why that works that way?" type of questions. We found this book to have good technical depth without requiring you to be a C++ guru to understand it.
VB COM - another excellent overview of COM, written from the perspective and point of reference of someone who already knows VB.