Cover says this is genre defining graphic novel. I doubt anything this labour intensive can become genre. Art is the labour intensive part. Script probably didn’t take much more time than as many pages of regular mass-produced superhero comic. Dave McKean’s art is the genre defining part if there is any. This kind of art is rare not unique. It is even rarer on superhero comic than in “art comics” if there is such genre.
Dave McKean is best known for Sandman’s covers. If you know those you have some image what is in this comic. The art is really ambitious and experimental. Maybe bit too ambitious and experimental. Comic gets hard to read when pages create whole spread art pieces with comic panes added to them. Comic panels are bit hard to read but they hinder the art piece. Joker’s dialog is hard to read thanks to how it is written. Other character have closer to normal speech balloons. This is easier to read as digital and with guided view. You miss some of the whole spread art but can follow the story better. 25th Anniversary Edition isn’t best in this because some of the art requires bigger resolution than it has. But I rather read it that the printed 15th Anniversary Edition I also have. There is newer edition with remastered art which might be better. Resolution issue isn’t big but it is noticeable few times.
Most of the time art is amazing. With guided view you don’t see upcoming panels which give better impact when you read the comic. Making whole spread art hinders phasing of panels. Junji Ito is master of putting twists after page turns. This one doesn’t have those. Kind of interesting this works better on format which wasn’t available when comic was released. Seeing only one panel at the time give you time to admire each panel.
Story is dream like journey into Arkham Asylum. It shows founder of Arkham Asylum Amadeus Arkham in flashbacks and Batman in current time going into Arkham Asylum to save hostages. Stories connect in the end hinting Arkham Asylum makes inhabitants insane instead of healing them. You should think this as a dream. Otherwise story doesn’t make too much sense. Story and art supports that interpretation.