The Untime & The Untime Revisited: Two Notebooks of M. Jules Gauthier, Journalist of Paris

The Untime is a state beyond space and time, discovered by Professor Rémy Lamartine in the 1890s. His daughter Agathe, and her beloved Jules Gauthier, a reporter on a Paris magazine, become concerned when Lamartine goes missing. Inside the Untime, they discover horrors beyond our imagining - hideous creatures which can destroy the sanity of their victims, and which seem intent on ruling our world.
Accordingly, they reluctantly return from the Untime and destroy the mechanism that enabled access. A year or so later, Gauthier and Agathe are married, but a strange outbreak of lunacy is reported. The newly-weds make their way back to the Untime, where they discover more surprises, and more dangers in this mysterious bridge between space and time. Gauthier tells his story in Gallic style, and while he is not a scientist, he provides explanations of the principles of the Untime as they were explained to him by Lamartine and his friend at the Sorbonne, Professor Schneider.
A captivating series in the tradition of Jules Verne and H. G. Wells, with a dash of H. P. Lovecraft stirred into the mix.