The Hundred Thousand Kingdoms, by N.K. Jemisin.
Yeine’s mother was the king’s daughter who had been exiled when she married a barbarian and gave up her birthright. Yeine has been raised to be a warrior of her father’s barbarian tribe. Now nineteen years old, she’s been summoned by the king, her grandfather, to the city called Sky, at the heart of the Hundred Thousand Kingdoms. The king has surprising news for her. He has declared her one of his heirs, and only one can inherit the throne.
This book was recommended when someone asked for something besides the usual “warmed over Celtic fantasy or novel from a Tolkien wannabee.” I realized then that that was also my reason for not reading much fantasy, so I gave it a try. I liked it a lot. It was entertaining, surprising, and I kept wanting to turn the page to find out what happened next. I need to read more Jemisin.