L space connects all libraries in the omniverse, as I understand it. so no, they don't need to share universes for the librarian to appear.

Also I recall that being the intended reference ITEHATTSD was making.

Ah, that is the Great A'Tuin, on whose back stand the four great elephants, Berilia, Tubul, Great T'Phon and Jerakeen, who bear the titular Discworld upon their shoulders.There used to be a fifth elephant, but it had a small whoopsie and is now the source of most of the fat and treacle that is mined by the dwarfs and humans across the Disc.

The Discworld is the setting of Terry Pratchett's Discworld novel series. Now normally I would describe what the series is about; but after the first few novels, each book is pretty much it's own self-contained story.

As you can probably tell, the setting itself is fairly comedic and light-hearted for the most part, but also tends to deal with a lot of philosophical concepts and serious social questions as well.

Everyone has their favourites, but if you wanted to try and get into it yourself,I would personally suggest either Small Gods, Mort or Men-At-Arms as your first reads.

Small Gods is a good one, as it is completely self-contained and you don't need to know any background info going in. Mort and Men-At-Arms are also fairly important as they serve as introductions to the two most recurring characters throughout the series, which are DEATH and the Anhk-Morpork City Watch respectively.