Lending it to a friend or member of your gaming group won't get you in trouble. Making a copy for all of your friends probably won't either because who is going to know? Making it publicly available is where it gets sticky.
It's a question of how tolerant the owners of the copyright are. Comic artists regularly produce commissioned work of characters that they don't own and if Marvel or DC wanted to drop the hammer on the artists they could but generally don't (although I know an artist who did receive a cease and desist letter from DC for selling cheesecake illustrations on eBay). Star Trek and Star Wars fan films fall into the same boat although Paramount/Star Trek changed its policy because of Axanar
Copyright law is a mess. I wouldn't be surprised if some of the monsters in the OP's book were created by people who never had another writing credit (or very few) and so tracking the original creators or their estates would be next to impossible so the work can't be legally reproduced for distribution. There are literally millions of orphaned works
out there that can't be used by the general public even though the rights status can't be determined.