The Empire of Dreams by Rae Carson

Even though Red Sparkle Stone is a foundling orphan with an odd name and a veiled past, she’s about to be adopted into the royal family—by Empress Elisa herself. Sixteen-year-old Red can hardly believe her luck. Then, in a stunning political masterstroke, the empress’s greatest rival blocks the adoption, and Red is left with no family and no future.

Grieving and lost, but determined to find her place, Red hatches a daring plan: she will prove herself as a recruit for the world’s most elite fighting force, the legendary Royal Guard—something no woman has done before. But it’s no coincidence that someone wanted her to fail as a princess, someone whose shadowy agenda puts everything she loves at risk. As danger closes in, it will be up to Red and her new friends—and maybe some new enemies—to save the empire. If they can survive recruitment year.