Book Girl and the Undine Who Bore a Moonflower (light novel)

"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?