Absolutely. The sequel books are a bit of a let-down, until you reach the final one of "The Adversary Cycle", NIGHTWORLD which is every bit as good as THE KEEP.
Interestingly, Wilson manages to link in his other works, so much so that some sites have made THE TOMB (now republished under it's original title RAKOSHI) and THE TOUCH part of the series - characters and ideas from what are strictly speaking stand-alone novels appear in the final "Adversary" book.
And yeah, it's not giving anything away to tell you that both Glaeken and Rasalom are in it...
I just re-read the book. Would love to see this on DVD. It was announced once, (quite a while ago now), then Paramount reneged on putting it out. It's about time they finally went through with releasing it.
What a coincidence, I'm about to watch this and there's a thread on it. The film is more than a little bonkers but I like it, a lot more than I did Mann's last film, Miami Vice.