I only started reading them a few months ago and am really enjoying them.
I've read:
RELIC
RELIQUARY
CABINET OF CURIOSITIES
RIPTIDE
THUNDERHEAD (about ten pages from the end right not)
I bring them up because many of them take place in New York City and are loaded with factual information about the city's streets and buildings. As a native New Yorker this brings an added dimension of fun, and also strongly reminds me of FPW's similar, lovingly detailed locations -- so I feel like the stories take place in the RK universe, even though Jack himself never makes an appearance.
The books are also similar in that they're loaded with protagonists who have far more initiative and intelligence than the people around them, and as a result get more involved with strange goings-on and as a result are able to save the world.
A main, recurring character throughout a subset of the books is Agent Pendergast of the FBI, a mysterious fellow who seems empowered by the Agency to investigate just about anything that strikes his interest. He's a great character and I look forward to reading the subsequent books that apparently tell more about him. I'd love for he and Jack to meet up in some cross-over story!
FPW, if you're reading this, I'm curious if you've ever met Preston and/or Childs and if anything fun was said!
-Oss