The Devil Makes Three — Tori Bovalino

When Tess and Eliot stumble upon an ancient book hidden in a secret tunnel beneath the school library, they accidentally release a devil from his book-bound prison, and he'll stop at nothing to stay free. He'll manipulate all the ink in the library books to do his bidding, he'll murder in the stacks, and he'll bleed into every inch of Tess's life until his freedom is permanent. Forced to work together, Tess and Eliot have to find a way to re-trap the devil before he kills everyone they know and love, including, increasingly, each other. And compared to what the devil has in store for them, school stress suddenly doesn't seem so bad after all.