I got this from reading an Amazon page about 'stories like Saw or Cube'. Someone said this was the original story in that vein. Interesting story; Philip K. Dick has some great plots. If I don't get to him this year, maybe next year will be my PKD sci-fi year.
Interesting. Someone on Amazon said this was where Hollywood got the idea for The 13th Floor. I think I've seen that, but I'll have to watch it again, just to be sure. :-P