To a Darker Shore by Leanne Schwartz

When her best friend is sacrificed to the devil, she'll go to hell and back for him
Plain, poor, plus-size, and autistic, Alesta grew up trying to convince her beauty-obsessed kingdom that she's too useful to be sacrificed. Their god blessed their island Soladisa as a haven for his followers, but to keep the devil at bay, the church sends a child sacrifice to hell's entrance every season-often poor or plain girls just like Alesta.
With a head full of ideas for inventions, Alesta knows her best shot at making it to adulthood is to design something impressive for the festival exhibition so she might win a spot in the university-acceptance could guarantee her safety. But Alesta's flying machine demonstration goes awry, a failure that will surely mean death. What happens is worse: Her best friend and heir to the throne, Kyrian, takes the blame expecting leniency but ends up sacrificed in her place.
To stop the sacrifices forever, Alesta plans to kill the monster that killed her friend. Prepared to save her kingdom or die trying, she travels to the depths of hell only to find Kyrian-alive, but monstrously transformed.
There is no escaping hell or their growing feelings for one another, and the deeper they descend into hell, the closer they come to uncovering a truth about the sacrifices that threatens to invoke the wrath of not only monsters but the gods as well.