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?