Book Girl and the Undine Who Bore a Moonflower

Book Girl and the Undine Who Bore a Moonflower ("文学少女"と月花を孕く水妖【ウンディーネ】) is the sixth book in the main series of the Book Girl light novels.

Back Blurb
"“I’ve been kidnapped by a bad person. Bring a change of clothes and your homework and come save me!” Duped by this seemingly earnest summons from Tohko, Konoha finds himself forced to spend his summer break at the Himekura villa, writing snacks for his greedy club president. But the shadow of a tragedy from eighty years past threatens to destroy their otherwise peaceful summer! What is the “secret” that stirs the Book Girl’s imagination?"