From the back cover:
Need a monster? Look inside, where more than 300 new pieces of full color art show what the monsters really look like!
This book contains more than 600 monsters, including all the creatures from the MONSTROUS COMPENDIUM(TM) Volumes 1 and 2! In addition, there are monsters from the other MONSTROUS COMPENDIUM Volumes, and some creatures never seen in the second edition AD&D(R) Game before.
Entries have been reorganized, corrected, updated, and fully cross-referenced, to make this the most valuable monster reference ever!