A fifth of the way into "Nightworld" by F Paul Wilson and it is now clear why The Adversary Cycle is six books and not four. You could probably read "Nightworld" as a stand alone book but you would be continually asking who is he , and what is that and how did that happen and how did he know that.
In fact there could be a prequel to "The Keep" but that is just something that stimulates the imagination.
Although "The Tomb" does not refer directly to anything in "The Keep" the links become clear in "Nightworld". It is similar with "The Touch" , again a sort of stand alone novel , but when you hit "Nightworld" it becomes clear that it is a part of The Adversary cycle.
"Reborn" and "Reprisal" do follow on from "The Keep" but not directly.
But essentially I now comprehend how the six books fit together and am rather impressed with how much I remembered and how much I had forgotten.
Unlike the dreams I have been having recently including a house with moveable showers filled with ice, and driving in a large van around somewhere that may have been Preston or somewhere in Scotland but I was unable to go in the direction that I needed due to roads being blocked or bricked up.
But I am not even going to try an analyze that as I have better things to do with my time.
Music wise we'll go with "Hole In The Ground" by The Mekons which is sort of appropriate for "Nightworld".