City Of Blades by Robert Jackson Bennett

In this dark and gripping fantasy sequel, a retired general is called back into service to investigate the disappearance of a government agent in a city that was once the heart of a magical civilization. The city, now ruins from a devastating war, holds a dangerous secret that could change the world—a miraculous technology that once powered an entire civilization, now sought by various factions for their own purposes. As the general delves deeper into the mystery, he encounters old adversaries and unearthly forces, and he must confront his own demons and the cost of his past actions while trying to prevent a cataclysm that could once again engulf the world in conflict. The 12174th greatest book of all time