I spent the first 8 years of my software engineering career (in the 80s) working under Dr John McDermott on XCON (then the world's largest expert system) at DEC's AI Technology Center. Required reading for being part of his team was this seminal work that I'm happy to say is now freely available from Columbia University.
Rule-Based Expert Systems: MYCIN
I urge you to read it. It will likely answer almost all your questions and probably give rise to many more.