Princeps by L. E. Modesitt Jr.
Now second only to the governor, and still hiding his powers as an Imager, Quaeryt is enjoying his new position, as well as his marriage to Lord Bhayar's youngest sister, Vaelora, when a volcanic eruption devastates the old capital of Telaryn. He and his wife are dispatched to Extela, Telaryn's capitol city, to replace the governor killed in the eruption. Not only must Quaeryt and Vaelora restore order to a city filled with chaos and corruption, but they must do so quickly. The regiment under his command (and the sources of his political power on the spot) must soon depart to bolster Telaryn's border defenses against a neighbouring ruler who sees the volcanic devastation as a possible opportunity for invasion and conquest.