This is outstanding! The ogre, owl bear, and kobold illustrations are immediate all time favourites for me. And the invisible stalker is masterfully executed.
I have one suggestion for the next iteration: re-order a few of the monsters that are burdened with leading adjectives so they are grouped together rather than scattered throughout the collection. ie "dragon, black", immediately followed by "dragon, blue", "dragon, green", "dragon, red", and "dragon, white", all under "D" rather than having "black dragon" on pg. 12 and then "white dragon" on pg. 142. The references should be under "dragon" rather than under each colour. (Same goes for the various bears, giants, etc etc).