Back in 2013, Pashaimeru created this handbook and put it on the TFGS forums. In the interest of providing resources as widely as possible for the remastering, it is mirrored on this site.
The information in the handbook describes the world of Pandemonium, the other planes of existence around it, and of course, the various monstrous denizens.
You can get it by clicking here. (Mediafire)