I would highly recommend any and all of Christian Eichhorn's Mörk Borg supplements - their quality is some of the highest of any you can find.
Monsters! is one of the more unique of Christian's collection, both in style and substance. You won't find other gameplay additions like encounters, dungeons, classes, or the like ... It's a bestiary full of enemies and monsters on offer! They're fantastic, and the Undead Generator is a great addition as well. Waclaw Traier's signature artistic style - while different than the sharp lines and bright color palettes you'll be familiar with in the core Mörk Borg book or other parts of Christian's collection - is phenomenal.