North Is the Night by Emily Rath

Two bold young women defy the gods and mortals, living and dead, in this darkly mythical, Finnish folklore-inspired fantasy duology.

In the Finnish wilderness, more than wolves roam the dark forests. For Siiri and Aina, summer’s fading light is a harbinger of unwelcome change. Land-hungry Swedes venture north, threatening the peace; a zealous Christian priest denounces the old ways; and young women have begun to disappear.
Siiri vows to protect Aina from danger. But even Siiri cannot stop a death goddess from dragging her friend to Tuonela, the mythical underworld. Determined to save Aina, Siiri braves a dangerous journey north to seek the greatest shaman of legend, the only person to venture to the realm of death and return alive.
In Tuonela, the cruel Witch Queen turns Aina’s every waking moment into a living nightmare. But armed with compassion and cleverness, Aina learns the truth of her capture: the king of the underworld himself has plans for her. To return home, Aina must bargain her heart—as Siiri plots a daring rescue of the woman she loves the most.