Evil Librarian is devilish fun, perfect for a witchy October or for teens who like their demons lighthearted. Suitable for middle school or high school in this parent’s opinion, the writing is smart, clever, and, best of all, threaded with musical theater references.

The story opens on Cynthia Rothschild’s senior year in high school, where her main concerns are her best friend, Annie, who is besotted with the new librarian; Ryan Halsey, the most beautiful boy in school, both a star athlete and a musical theater star; and the school play, the delightfully ironic “Sweeney Todd: The Demon Barber of Fleet Street.” This musical thriller about revenge, blackmail, and murder provides just the right undercurrent of strangeness and metaphor.

The very grounded and pragmatic Cynthia, or “Cyn,” is stage manager of the school’s major musical production. She admits that she doesn’t want to be “that girl, the girl who thinks all she needs to be happy is a boyfriend.” But part of her yearns for one.

In other, lesser YA novels, this tension would be enough to propel one through the entire story. But Evil Librarian has plot twists galore before we get to the easy stuff of having a boyfriend. There are demons in the school who will soon be threatening students, teachers, and the principal (and even one another in demon-versus-demon battles around the building).

Luckily, Cynthia is the rare person who has an innate shield against the dark forces. Even more fortunate, her school has been taken over by demons enthralled with musical theater, especially “Sweeney Todd.” Cynthia ends up making a bargain with the devil — aka Mr. Gabriel, the school librarian — to continue the year and the play.

After some interactions with various ghouls, and after hilariously invoking both the Almighty and composer Stephen Sondheim in one breath, Cynthia eventually succeeds at the mantra of every theater-lover: the show must go on. And the show does go on in a spectacularly entertaining conclusion to the novel.

Knudsen captures a young person’s love of musical theater, particularly working on a major high-school production, and couples it with a twisted farce of demons on and off stage.

My one quibble with this entertaining novel is its title character: I wish he were more of a librarian and talked about books, spouted literary quotes, liked research and footnotes, or took on the nerdy-wonderful characteristics of most librarians I know. Why didn’t the author make him an evil English teacher, or better yet, an evil drama instructor?

He ends up somewhat on the periphery of the story, a MacGuffin of sorts, who sets our likeable, unstoppable protagonist on her way to saving her best friend, the school, and the big show.

Ultimately, however, kudos to Knudsen. When your teen is ready for a clever dramedy with a bit of romance and a dash of demons, Evil Librarian should do the trick.

Caroline Bock is author of the critically acclaimed young adult novels Before My Eyes and LIE.