Cancelled
It was a dark and stormy night... Wait, no, that's not right... Hold on a second... Ah here we go.
It was just another normal(1) day in the Unseen Library, until L-space opened up and sucked the Librarian into a different universe populated by pastel ponies.
Meanwhile, in a pastel castle on a pastel mountain, three princesses happened to be in the wrong library at the wrong time and, well, I think you see where this is going.

(The author hereby warns the reader that to expect anything approaching a coherent story or plot structure is pure folly... or in his own words. "I'm doing it for the Lolz.")

For your listening pleasure

(This is a comedy story using Discworld and its inhabitants(2) along with Equestria and its inhabitants(3).)
(1) For any given value of 'normal'
(2)Owned by Terry Pratchett
(3) Owned by Hasbro
(I am aware of the other, kinda similar, crossover.)