Henry Kuttner 
Well of the Worlds 

When the curiously exotic millionairess Klai Ford started telling him about ghosts in a uranium mine, Sawyer knew he’d better be ready for anything in his investigations. But he didn’t count on being drawn into a passage between dimensions and tossed adrift in a world of islands floating in the sky, where strange brute-like creatures were attacking the cities in a vast struggle for power. Lost in this new world, Sawyer realised that the key to everything lay in the mysterious Well of the Worlds — and that the future of the universe lay in its secret.