Quoted from dmarston:
To be more exact, the Pinball series that began with Pinball 1 was announced as a 10-volume set from Coin Slot Books. I think that was still the plan as late as 1987. When he switched the publishing, the Encyclopedia was also initially described as a projected 10-volume set, but Bueschel later said it would be scaled down to 6 volumes. I'm sure that he had outlines for all the volumes; that's the way he worked.
Is Gordon still working on this set or has it just been shelved forever?