«The Book of Forbidden Wisdom» by Gillian Murray Kendall

In a Venn diagram of Jane Austen, Mary Robinette Kowal, and Marie Brennan, you'll find Gillian Murray Kendall's fantasy-of-manners, The Book of Forbidden Wisdom.
In a world of blood and betrayal, love is the only redemption.  But that knowledge can only be reached by means of magic and a journey, by way of a confrontation with feelings that are hard to understand—or bear.
On Angel’s sixteenth birthday, her younger sister, Silky, wakes her to prepare her for a marriage to Leth, a man she likes but does not love. Trey, her oldest childhood friend who is secretly in love with her, watches helplessly.
But Angel’s brother, Kalo, interrupts the wedding ceremony. He wants her dowry, and he also believes Angel can lead him to The Book of Forbidden Wisdom. In a world where land is everything, this book promises him wealth. In the night, Kalo goes to Angel’s room to threaten her, but Trey has rescued both Angel and Silky, and the three of them—joined by an itinerant singer—themselves seek The Book of Forbidden Wisdom. While Kalo believes the book contains land deeds, they believe it harbors great power.
Always just a step ahead of Kalo, Angel, Silky, Trey, and the Bard finally arrive at the place of The Book. But things have changed now:  Angel knows her own heart at last.  Confronted by evil, at the end of the known world, Angel and her companions turn and fight. Together. And in so doing, they find that love contains a power of its own.