Nightworld. I've re-read this book more than any others in F. Paul's collection (I usually re-read very few books, btw). I liked it so much it inspired me to use it as a background for a AD&D adventure back in the day.

Since I didn't have any stats of the beasties in Nightworld, I had to improvise using the AD&D Deities & Demigods Cthulhu Mythos creatures. I kept the descriptions the same as in Nightworld, however.

Needless to say, it scared the living sh*t out of my friends (who all thought they were macho studs at the time).

http://www.scribd.com/Eniena/d/6044909-A...ock-Mythos

The Keep. It was my first FPW book I read when it first came out and I have read it over and over, just an amazing book.

Glaeken is my favorite character, I like Jack, but think of the history Glaeken has seen, of the things he has done. Would love to have more books written on him.