Welcome to the board, Carcosa! In answer to your question I have cut and pasted from Aphew's FAQ page loctated at
http://webpages.charter.net/mjturman/fpwfaq/rj3.htm:
"At the time FPW was finishing up the Adversary cycle, he did not know he was going to write a whole new series of Jack books. A few years after NIGHTWORLD was published, FPW decided to write more Jack. However, considering the apocalyptic nature of Nightworld, he could hardly set his new Jack books after that novel. Therefore he decided instead to set the books in the time between THE TOMB (the first Jack book) and NIGHTWORLD.
"This gets very sticky because FPW threw in quite a few pop culture refs and definite dates into the Adversary Cycle (which ends in the early 90s), yet has set the Jack books in our current time. Thus many gentle readers are confused mightily when Jack doesn't know things he should in Nightworld, or is still running around eight years after the world has pretty much ended. This problem has been somewhat mitigated by the revised version of The Tomb (Tor/Forge, 1998), in which pop culture refs were updated. When FPW is done with the latest round of Jack books, he plans to go back and revise Nightworld so that it meshes better with the current state of Jack's knowledge and adventures. Currently this revised version of Nightworld is scheduled to be published by Borderlands Press in 2006."
Hope that answers your question.