Warrior Queen by Fil Reid

It’s been almost three years since Gwen, a 21st century librarian, fell back through time to become King Arthur’s Queen Guinevere. Arthur’s drawn the legendary sword from the stone and beaten his jealous brother. The only thing left to do is prove to his fellow kings that he’s worthy of the title High King of all Britain. Not an easy task.
At the same time, the countdown through the list Gwen knows of Arthur’s legendary twelve battles is flying past, bringing the thirteenth, fatal Camlann, ever nearer. With history tracking so close to legend, all of Gwen’s fears seem likely to come true.
While Arthur is away fighting Irish invaders in the far west, messengers arrive from the north, requesting urgent help. Gwen is forced to lead a party of her husband’s men across the River Severn in search of Arthur’s army. But she’s heading towards the inevitable realization that there are no true victors in war.