Book Girl and the Corrupted Angel

Book Girl and the Corrupted Angel ("文学少女"と穢名の天使【アンジュ】) is the fourth book in the main series of the Book Girl light novels.

Back Blurb
"With college exams approaching, Tohko Amano — president of the literary club, closet book-eating goblin, and shameless procrastinator — does the unthinkable and declares club activities suspended! Unencumbered by the demand of his taskmistress to deliver handwritten improv stories, Konoha finds himself helping his oft-estranged classmate, Nanase Kotobuki, in the music room after school. When one of Kotobuki’s friends goes missing before Christmas, though — vanishing amidst rumors of her being an “Angel of Music” — Konoha finds himself swept up in a mystery unfolding as if from the pages of Gaston Leroux’s seminal work…"