The Untime Revisited: A novel of 19th-Century Paris. The Untime, #2

On his return from the Untime, as described in the previous volume, Jules Gauthier marries the orphaned Agathe Lamartine, whose father died horribly in that mysterious state between time and space. But. is Agathe really orphaned? An outbreak of nightmares in Paris, leading to madness of the dreamers, persuades Jules and Agathe that Lamartine is still alive, trapped in the Untime, together with monsters intent on taking over our world.
Summoning all their courage, Jules and Agathe, accompanied by the faithful Louis, and supported by the irascible Professor Schneider, return to the Untime to seek Lamartine, and defeat the horrifying creatures that exist there. But all is not as it seems.