The Keep is one of my favorite books of all time and the rest of his stuff is really great.
Here's the order to read:
THE KEEP (Adversary Cycle)
REBORN (Adversary Cycle)
REPRISAL (Adversary Cycle)
THE TOMB (Adversary Cycle/Repairman Jack #1)
LEGACIES (Repairman Jack #2)
CONSPIRACIES (Repairman Jack #3)
ALL THE RAGE (Repairman Jack #4)
HOSTS (Repairman Jack #5)
THE HAUNTED AIR (Repairman Jack #6)
GATEWAYS (Repairman Jack #7)
CRISSCROSS (Repairman Jack #8)
INFERNAL (Repairman Jack #9)
HARBINGERS (Repairman Jack #10)
BLOODLINE (Repairman Jack #11)
THE TOUCH (Repairman Jack #12)
BY THE SWORD (Repairman Jack #13)
GROUND ZERO (Repairman Jack #14)
FATAL ERROR (Repairman Jack #15)
DARK AT THE END (Repairman Jack #16)
NIGHTWORLD (Adversary Cycle)