Yami to Boushi to Hon no Tabibito

A high school student named Hatsuki lives with her older sister, Hatsumi, whom she deeply admires. On Hatsumi's 16th birthday, a green light surrounds her, and she vanishes before Hatsuki's eyes. With the aid of a creature that looks like a plump baby chick, Hatsuki pursues her sister and arrives at a location known as "The Great Library," where countless worlds are contained within books. However, Hatsumi is not there, prompting Hatsuki to begin a journey through these books in search of her missing sister.