Long ago, the keepers of a Library of Alexandria received word just in time that the library was to be burned. Rather than see it and all its knowledge and magic destroyed, they preformed a powerful ritual, which cut it out of the world it came from to send it spiraling out into the wild multiverse.
In time, the Library learned to reach outside itself, connecting to the doors of buildings like the one it had once been housed inside. Soon, it began to receive patrons, people as lost as it, and people whose questions its contents could answer. Some of these people found what they were seeking and then left, some remained for long periods of time, learning all that was within, while some? Some looked upon the wealth of knowledge with greed in their eyes.
Bit by bit, books were taken from the Library, and as its collection dwindled, so did it. Fearful, the Library hatched a plot - it searched for one of those lost persons, a lover of books, a caretaker, a Librarian, to care for it and seek out those books which had been stolen.

The Librarian did much for the Library; they reclaimed much of the collection, and brought in new books to replace those lost, and more. They discovered new ways the Library could twist its magic, allowing it to spend great sums of power to snatch up whole libraries doomed to the pyre or lost as it was. They visited far off worlds, learning new things, and writing books to add to the collection.
But eventually, they grew old, and tired, and they did not wish to live forever. However, not wishing to leave the Library alone, they created a book. The Book of Alexandria, with which future Librarians could bond with the Library, and learn how to use the power it grants to care for it and assist those who lose themselves inside of its walls.
This done, and their duty complete, they laid down in the depths of the Library, and they died.

In dying, they left the Library grieving and hollow and lost. In denial, hoping against hope that its Librarian would return, the Library slowly wasted away rather than search for someone else, allowing its collection to once more be stolen, its insides to collapse and rot, until all that was left was a ruin, and its power dwindled to nearly nothing.
Finally, as the Library was left with only the Book as a source of power, only a breath away from death... it reached out once more, in simultaneous hope and despair, beckoning another lost soul, a lover of books and knowledge, a caretaker, to take up the mantle once more.

And so it has been for untold eons, a cycle, the Book changing hands as the Library loved, lost, and was remade...