Yamibo - Darkness, the Hat, and the Travelers of the Books

Hatsuki is a high school student who lives with her sister, Hatsumi, and secretly harbors strong feelings for her. On Hatsumi's sixteenth birthday, a mysterious green light envelops her, and she vanishes right before Hatsuki's eyes. Desperate to find her, Hatsuki follows with the help of a strange, chubby baby chick-like creature. They end up in a place known as "The Great Library," where countless worlds exist within books. But Hatsumi isn't there, forcing Hatsuki to embark on a journey through these book-bound worlds in search of the sister she loves.