The Game (The Emperor's Library: Book Three) - The Emperor's Library, #3

The Game, Book Three of The Emperor's Library, takes place twenty years after the events recounted in The Tritargon. When the story opens, Jon is living quietly in Kar, having withdrawn from the heroic stage he once occupied. With Dan, his lover and business partner, he deals in relics of the Empire, until one transaction puts them in possession of a mysterious book sought both by The Chosen, now rulers of the city, and by a dissenting sect that seeks to overthrow The Chosen and their leader.
Caught in the struggle between these forces, Jon must reenter the larger world he had left behind and once again risk his life for a cause. With the help of Krator, a depraved mathematician, he attempts to solve the riddle of the game, the long-lost key to an ancient science that can transform-or destroy-the planet. Jon's quest reunites him with Zoë, Klei and other figures from his past from whom has been long estranged.
Together, they find themselves in a multidimensional confrontation that none of them could have anticipated. All who take part in the final game find their lives irrevocably changed.