To a Darker Shore by Leanne Schwartz
A da Vinci-esque teen girl inventor on a Dante-esque journey through hell to save her best friend.
Plain, poor, plus-size, and autistic, Alesta grew up trying to convince her kingdom that she’s too useful to be sacrificed like so many of their country’s poor to appease the infernal monster across the poison sea in hell.
When Alesta’s attempt to prove herself with inventions goes awry, her best friend and heir to the throne, Kyrian, takes the blame expecting leniency―and ends up tithed in her place.
To end the sacrifices forever, Alesta plans to kill the monster that killed her friend. She travels to the depths of hell only to find Kyrian, alive, but monstrously transformed.
There’s no escaping hell or their deeper feelings for one another, and the farther they go, the closer they come to uncovering a truth about the tithings that threatens to invoke the wrath of not only monsters but the gods as well.-Goodreads