The Indigo King — James A. Owen

"Answer the question unanswered for more than two millennia, and perhaps you may yet restore the world."
On a September evening in 1931, John and Jack, two of the Caretakers of the Imaginarium Geographica, discover a cryptic warning on a medieval manuscript -- which is not only addressed to them but seems to have been written by their friend, Hugo Dyson! But before they can discover the origin of the strange book, Hugo walks through a door in time -- and vanishes into the past.

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 both worlds have fallen into darkness under the reign of a cruel and terrible king.

The only hope to restore the proper order of things lies on a forgotten island at the edge of the Archipelago, where a time travel device left by Jules Verne must be used to race through history itself -- from the Bronze Age to ancient Alexandria and the founding of the Silver Throne -- in one night. And in that single night, John and Jack will find that the only way to save their friend and stop the chaos destroying the world is to solve a 2,000-year-old mystery: Who is the Cartographer?