Yami to Boushi to Hon no Tabibito

Yami to Boushi to Hon no Tabibito (The Darkness, the Hat and the Traveler of Books) is a hentai visual novel game and non-hentai, 13 episode anime series. They follow the travels of Hazuki, a quiet high-school girl, as she searches for her vanished older sister Hatsumi throughout many different "book worlds." She is accompanied by the strange caretaker of the book worlds' library, Lilith- who wears the hat referred to in the title, and what a hat it is- as well as a talking parakeet, Ken-chan.

They are most popular among shoujo-ai fans, as a majority of the female characters are blatantly attracted to Hatsuki.