Yami to Boushi to Hon no Tabibito
English title: Yamibo: Darkness, The Hat and the Travelers of the Books
Japanese title: ヤミと帽子と本の旅人
Type: TV
Episodes: 13
Premiered: fall 2003
Source: Visual novel
Genres: Adventure, Fantasy, Girls Love, Mystery, Ecchi
Studios: Studio Deen
Duration: 24 min per ep
Rating: R+ - Mild Nudity

Hatsuki is a highschool student living with her sister, Hatsumi, who she has a huge crush on. On Hatsumi's 16th birthday, she is suddenly surrounded by a green light and disappears in front of Hatsuki. She manages to follow Hatsumi with the help of a being resembling a fat baby chick (literally), ending up in a place called "The Great Library", which is full of different worlds stored in books. Hatsumi wasn't there, though, so the search for Hatsuki's great love begins and involves traveling from book to book.

(Source: AniDB)