The Tomb and The Touch were initially written as stand-alone novels; it is only in Nightworld that they were retroactively made part of the Cycle as their respective protagonists all come together to fight the final battle against the Otherness.
Although these six books make up the "core" of "The Adversary Cycle", many more novels and short stories tie into the series. For example The Tomb is also the start of the Repairman Jack series of novels, which quickly became deeply entwined with the Adversary Cycle. They take place in the last three years before Nightworld and include some of the main characters from the original Adversary Cycle. This is all part of the "Grand Unification" theme of Wilson's work.