The Thousand Names by Django Wexler is book one of The Shadow Campaign, out next month from Del Rey (£16.99):
“Captain Marcus d’Ivoire, commander of one of the Vordanai empire’s colonial garrisons, was resigned to serving out his days in a sleepy, remote outpost. But that was before a rebellion upended his life. And once the powder-smoke settled, he was left in charge of a demoralized force clinging tenuously to a small fortress at the edge of the desert.
To flee from her past, Winter Ihernglass masqueraded as a man and enlisted as a ranker in the Vordanai Colonials, hoping only to avoid notice. But when chance sees her promoted to command, she must win the hearts of her men, and lead them into battle against impossible odds.
But the fates of both of these soldiers, and all the men they lead, depend on the newly arrived Colonel Janus bet Vhalnich, who has been sent by the ailing king to restore order. His military genius seems to know no bounds, and under his command, Marcus and Winter can feel the tide turning.
But their allegiance will be tested as they begin to suspect that the enigmatic Janus’s ambitions extend beyond the battlefield and into the realm of the supernatural — a realm with the power to ignite a meteoric rise, reshape the known world, and change the lives of everyone in its path.”
This is termed “Flintlock Fantasy” I gather...