Yami to Boushi to Hon no Tabibito

Yami to Boushi to Hon no Tabibito (Yami, the Hat, and the Book Travellers), a.k.a. Yamibou

The Great Library: A place where every world is maintained within books. By and large, people can live out their entire lives without ever knowing it exists.

Azuma Hazuki was living out her life as a fairly Ordinary High School Student in one of these worlds, until her adopted sister, Hatsumi, disappeared at midnight on her sixteenth birthday. Intruding immediately on her shock was a bird-like being called Ken-chan, which had been sent by its master to find someone called Eve...

Yamibou is, simply, the story of the people whose lives were touched by Eve as she travelled through the worlds contained in the books of the Great Library, and their continued attempts to find her. From Lilith, the current guardian of the Great Library and caretaker of its books (that's supposed to be Eve's job); to Gargantua, whose life was forever altered when he witnessed Eve's apparently magical powers during her stay in his world; and finally to Hazuki, who loves her adopted sister and sought to find her at any cost; each had their own reasons to search.

Many have called Yamibou a watershed moment for serious schoolgirl lesbian moments in anime (though it's been played as comedy since at least 1999 in Steel Angel Kurumi, and probably earlier), the recent rise of yuri in anime can easily trace its beginning to the late-2003 run of this show. Humorously enough, many of the people who watched the show for just that reason, and who had glowingly referred to it at the defining shoujo-ai series, absolutely despised the ending. (Several have since attempted to pass the mantle of "defining yuri series" to Kannazuki no Miko.)

Yamibou is based on an "H"-game by Root with artwork done by Carnelian. The original game's plot is vastly different, lacking much of the yuri due to starring a nameless male main character (who was completely eliminated from the anime.)

This show provides examples of:
 * Airplane of Love
 * All Just a Dream (Hazuki and Hatsumi share a very passionate scene, only to have it turn out to be nothing more than a dream--in the new world at least.)
 * Alternate Continuity (the series itself, compared to the game by Carnelian)
 * Anachronic Order (the first episode is broken into two parts, depicting the very start of Hazuki's quest and a later adventure within it)
 * Ancient Keeper
 * Bait and Switch Lesbians
 * Because Destiny Says So (Eve's continued disappearances--though this may not be "destiny" so much as a basic part of Eve's character, which makes it all the more infuriating.)
 * Bleached Underpants
 * Character Exaggeration (In the game, Lilith is an intelligent and cunning woman, if somewhat of a flirt. Only the 'flirt' part seemed to make the transition to the anime, leaving her a borderline bimbo with a lust for Hazuki that wasn't present previously--though even in the anime she hints at being more intelligent and compassionate than meets the eye now and then.)
 * Cipher Scything The nameless body-hopping protagonist of the game is replaced by the minor character Hazuki.
 * Commedia Dell Arte Troupe
 * Cute Mute (Hatsumi)
 * Dangerous Sixteenth Birthday
 * (At least in the anime.)
 * Even the Girls Want Her (Hazuki; however, if they're not Hatsumi, she's not interested.)
 * False Camera Effects
 * Fan Service (covering pretty much every sub-category, too)
 * Fractured Fairy Tale (One story involves a princess who must choose from a selection of princes who are, due to tradition, forced to dress up like frogs and hop around ...)
 * Friend to All Living Things (Quill)
 * Gainax Ending (fans are divided as to whether it's a Downer Ending or not)
 * Only because we don't have a 'bullshit! ending' trope...
 * It's actually three endings at once, none of which fit together. In one part, Hatsumi leaves a message, in the next, the message is entirely erased. Many fans nowadays seem to think the ending is supposed to be a Pick-and-Choose-your-ending thing.
 * Gaiden Game: Yamibous visual novel was given one in the form of Koge to Lilith no Nachtmusik. It's basically Lilith having lots of encounters with people in various outfits. Fridge Brilliance dictates that it's probably to make it easier to fit in wherever she goes...
 * Bonus points for some of those outfits being call-backs to some of the older games that Carnelian worked on, like KaoNoNaiTsuki and Machine Maiden.
 * Hair Colors
 * Hot Librarian (in a more literal sense)
 * Jigsaw Puzzle Plot
 * Joshikousei
 * Kansai Regional Accent
 * Kawaiiko (Lillith, among others)
 * Love Letter Lunacy
 * Magic Librarian
 * Magical Library: The Great Library consists of books that contain every single world of the multiverse down to the smallest detail.
 * Mayfly-December Romance
 * Medieval European Fantasy
 * Multiple Tailed Beast: It stars two kitsunes, one with nine tails and one with just two.
 * Nice Hat (right there in the title, and the picture)
 * Nubile Savage (Quill)
 * Ordinary High School Student (Azuma Hazuki)
 * Parental Abandonment (Hazuki and Hatsumi may well have parents, but they never appear, though Hazuki asks her mother a question in the final episode.)
 * Pillar of Light
 * Really Seven Hundred Years Old - Possibly. Fridge logic suggests Hazuki has spent an improbable amount of time in various books, and it may be more like seven million. Of course, time itself is up for grabs in this series.
 * Reincarnation
 * Schoolgirl Lesbians (At the time of its release, practically the Platonic ideal of Schoolgirl Lesbians. Wow.)
 * Shout Out: Leading to possible Epileptic Trees: Hazuki shares her last name with Io Azuma, a protagonist of ROOT's previous visual novel (also with Carnelian's art), Kao no Nai Tsuki.
 * Spell My Name With an "S" - Upon broadcast, the most commonly-viewed fansubbed version of the anime had some rather different ideas on name spellings. Among them were Marieru instead of Mariel, Garuganchua instead of Gargantua, Jiru instead of Jill, and most noticeably, Ririsu instead of Lilith. (Curiously, they did not render Adam and Eve as Adamu and Ibu.) A re-release by a different group in 2007 used the romanizations listed in The Other Wiki.
 * Super Deformed (Koge-chibi)
 * Tall, Dark and Bishoujo (Azuma Hazuki)
 * Theme Tune Cameo
 * Trapped in Another World In several worlds, actually.
 * Twelve-Episode Anime