The Servants of the Storm by Jack Campbell

A Storm that will wreck a world threatens Dematr. Only Mari, one of the Mechanics who control all technology through their Guild, has a chance to stop it. She and Mage Alain have survived numerous attempts to kill them and have gained many more followers, but the Storm of chaos, born of centuries of enslavement, grows ever closer.

Mari leads an army now. She and Alain must fight together to bind back the Broken Kingdom and build a force strong enough to defeat the might of the Great Guilds. But the Storm has many Servants who seek to preserve or gain personal power or wealth, or fear the New Day that Mari seeks to bring to the world.

And Mari knows that victory will mean nothing if the precious knowledge brought long ago to their world is destroyed. In order to save it, Alain and Mari will have to pierce through the heart of their enemies' power and confront once more a place of ancient nightmare.