The Devil Makes Three by Tori Bovalino

Tess Matheson only wants three things: time to practice her cello, for her sister to be happy, and for everyone else to leave her alone.

Instead, Tess finds herself working all summer at her boarding school library, shelving books and dealing with the intolerable patrons. The worst of them is Eliot Birch: snide, privileged, and constantly requesting forbidden grimoires. After a bargain with Eliot leads to the discovery of an ancient book in the library’s grimoire collection, the pair accidentally unleash a book-bound demon.

The demon will stop at nothing to stay free, manipulating ink to threaten those Tess loves and dismantling Eliot’s strange magic. Tess is plagued by terrible dreams of the devil and haunting memories of a boy who wears Eliot’s face. All she knows is to stay free, the demon needs her… and he’ll have her, dead or alive.