The Uncrowned Empress (The Magicians, #171)

At an intergalactic convention, an envoy from Alpha Centauri and his queen try to get a peace treaty with the magicians by the queen marrying the celestial emperor, who appears to be a widower.

He is and he isn't. He lost a 3rd wife, but not his second wife, who he is deeply in love with. But no one knows of her. They think she's the abandoned wife of his deposed emperor's grandson. Who he is. The queen catches the lovers together and blackmails the emperor into leaving his wife to marry he...