The books you've found in the chamber of Nerull have revealed some interesting truths.
First that Nerull and Boccob are actually the same god. Like two sides of the a coin. When in the unwatched he is Nerull when in the material plane he is boccob.
Second, people entering into the unwatched is exceedingly rare, nearly unheard of as are the horrors that exist there. Beings and monsters that only exist in nightmares.
Third, there are a few places where the unwatched directly touches the material plane and these locations are deemed holy by worshippers of Nerull. Also they seem to be the direct source of abberations in the world of Ebinon.
Fourth, one of these locations is suppose to exist near the town of Falidor.