Works of Grigore Summers
From Wulfgard Wiki
The many works of Grigore Summers include, but are not limited to, such works as Vita Mortis, primarily about the undead; the Caudex Daemonum, primarily about demons and witchcraft and other denizens of the Underworld; and Abominations of Chaos, which deals with monsters such as the Chaos Races.
Summers' work may exist within his many biases, but he remains one of the best and indeed one of the only sources of anything approaching information about various kinds of monsters and even some magic. For more information regarding the author, see the Grigore Summers article.