Gotta be honest this ending kinda sucked, don't get me wrong it was written really well and the art was really good to walk along side it but THE ACTUAL STORY that sucked. Ending the story at just chapter 27 is perfectly fine but that was not the direction that it should have gone.
So much build up just for you to think "ah so this is both the aftermath and where we uncover the big turning point of the whole story, this is the beginning of a great adventure everything changes from here out" just for that ending.
Despite having no connection to spirits, a crazy connection to demons, just watched someone get absorbed due to their foolishness, he's already neck deep in the whole demon business, have heard the life stories of the demons he calls friends from their biggest regrets to their strongest struggles, and that's a few of many things that I want you to keep in mind for what I'm about to say next: he quite literally gets the chance to change everything, to make this world a better place, he has seen first hand what is happening and he can change that, for all he knows it is quite literally too late to turn back AND HE SAYS NO??!? You could have had the perfect ending point with Norman "alright I've witness the horrors first hand and I'd rather they happen to someone else" the the next few scenes play out a little differently but mostly follows the same rhythm and still result in Norman leaving but this time to change the demonic realm (and maybe the human realm too): an adventure so Grand that it will be written in the history books. And he waves goodbye Ars Goetia in hand and maybe some of the known demons are ghostly yet in the frame. And instead of the last little snippet of it being what it was, it's a hooded man walking up to the same kid trying to do the summoning, the man scolds him because of how dangerous this is and says something along the lines of "let me teach you how to do this properly" and the next frame shows a older looking Norman holding the Ars Goetia probably with his hood pulled down and the same heart warming smile on his face, boom last page thank you for reading the Ars Goetia everything else is left to viewer interpretation and even leaves plenty of room for fan made continuation of the story
Now that I look at it rewritting an entire ending (especially without checking for errors) feels a tad disrespectful but you could just view it as my love for this story.