The End of Eternity by Issac Asimov (1955)
Eternity is a time traveling organization that tries to optimize human history.
They use a kind of time machine called a "kettle", which operates much like an elevator. Different historical epochs are like different floors. Naturally each era is different. In one, the style for room decor is to make all the walls and furniture like mirrors.
Eternity organization is located in a separate time continuum from the one that contains Earth. Eternity recruits apprentices from the Earth continuum and brings them into the Eternity continuum.