Lost Between the Pages - Third Page by Alice Beauverd

Joshua keeps diving into book after book, searching for Erin's sister and friends. He slips through the library mirrors and lands right in the middle of another story. This time? He's in a wild fantasy world, living as Skeja—a woman just drafted into an army of female knights under Crown Princess Eyhylde. Skeja's twin, Skura, joins up too. Right away, Skura notices something's off with her sister, but she blames it on nerves. Who wouldn't be anxious? Meanwhile, Joshua—stuck as Skeja—has to survive this brutal world and dig up any clues about where Erin's friends might be.