Borrowed Sun. Sun and Moon Series, #1

Liora has spent her life as the Veil's sharpest secret-trained to cut, taught to obey, and never allowed to choose. But when her sister is stolen into the thin places between worlds, Liora is forced into an alliance with Keiran, a prince of the Sun Court whose loyalty is cracking at the seams. To save Mara, they must cross into the Night Court-where bargains are currency, shadows have teeth, and an ancient debt-collector known only as the Toll begins to listen.
As enemies close in and truths surface about the Sun Court's "quiet work, " Liora discovers her voice can rewrite the ledger of power itself. at a price. In a world of dangerous doors and darker promises, love may be the only thing neither court can control.