The Indigo King by James A Owen

On a September evening in 1931, John and Jack, two of the Caretakers of the Imaginarium Geographica, discover a plea for help on an ancient medieval parchment--which seems to have been written by their friend, Hugo Dyson! When they rush to warn him, Hugo is abducted by fierce creatures called the Un-Men, who have mistaken him for the third Caretaker, Charles. And in that moment, the world begins to change.
The Frontier which separates our world from the Archipelago of Dreams has fallen. Dark and terrible beasts roam throughout England. No one can be summoned from the Archipelago. And worse, their mentor and ally Bert has forgotten them entirely!