The Key Masters of Telemma

Bill Newell and Carla Meadows, both seventeen and both dying, live out their final days in a palliative care center in Portland, Oregon. Bill has no hope of recovering, but the appearance of a strange key on his bed one day intrigues him. Carla receives the same key, and soon they find themselves in a pocket universe ruled by one being, a being-not-a-being named Malgorath who wields immense power.
Malgorath makes them a deal. Find four keys, ones of silver, gold, glass, and paper, and return them to him. Upon their retrieval, he will cure them of their illness and send them back to Earth. For Malgorath, the keys are the key to finding Falnarth, a mythical world that holds many secrets. Bill and Carla have no secrets to discover, save life. With no choice in the matter, they agree to the terms and soon find themselves journeying to worlds of silver, gold-ruled by robots-glass, ruled by a power-mad queen, and one of wood.
Each key's retrieval is more dangerous than the last but Bill and Carla are determined to succeed in their quest. However, they soon realize that Malgorath has his own agenda, and they must do everything in their power before the demon can fuse the keys together and destroy-and recreate-the universe.