Yami to Boushi to Hon no Tabibito  

Traveller of Darkness, Traveller Yami, Hat and Book, Yami To Boshi To Hon No Tabibito, Yami, the Hat, and the Travelers of the Books, YamiBou, YtBtHnT, ヤミと帽子と本の旅人

Hatsuki is a high school student who lives with her sister, Hatsumi. She has a massive crush on her sister. On Hatsumis 16th birthday, she disappears in front of Hatsuki surrounded by a green light. With the help of a creature that resembles a fat baby chick, Hatsuki is able to follow her sister and ends up in "The Great Library," which contains numerous worlds stored within books. Despite Hatsumi not being present in this library, Hatsukis quest to find her true love continues as she travels from book to book.