Piranesi by Susanna Clarke

Piranesi lives alone in a building of seemingly endless labyrinthine rooms filled with various statuary and an ocean floods the rooms periodically, but Piranesi understands the tidal patterns.
He is visited once a week by a man he calls "The Other" who brings him food and supplies and asks for his help in finding a secret knowledge hidden in the house. The Other warns him of another person he may encounter who could do him harm and he must avoid that person.