Tempering Earth by Reagan Claire

After discovering her true identity and coming to terms with an unimaginable heritage, Della Doe Deare finally thinks she has things figured out. She is learning the ropes of both her human, and non-human responsibilities; ones which will soon include leading an army, and may one day mean ruling a people. As the sole person able to control all five elements, she will have the power to protect everyone that she loves–and destroy anything that threatens them. But first, she has to survive the Elfennol training.

When an enemy with a reputation for torture sets his murderous sights on Della, she must submerge herself further into the Elfennol world, explore the other half of her inheritance, and move toward a destiny too large for mortal shoulders.

But not everything is as it appears; not all Clades are her enemies and the Elfennol aren’t as pure as they let on. When Della finds friendship – and protection – in the form of a troubled boy born on the wrong side of an endless war, she discovers that this new world operates only in shades of gray and the only difference between good and evil is which side you’re fighting for.

*****
This is the second book in the Gathering Water Trilogy, and should not be read out of order.