Alchemy Series by Douglas W. Clark

While the medieval town of Pomme de Terre suffers from a plague of demons and a baron determined to keep his subjects in ignorance, a pair of unlikely heroes - an aquatic alchemist and his disgruntled apprentice - trek across the land to discover the source of demonic infestation and bring enlightenment to their land. Clark's first novel is a tongue-in-cheek fantasy that explores the lighter side of the Middle Ages and uncovers the "true" origins of indoor plumbing.