Twilight Sparkle was granted a Royal Library as part of the castle in which she is residing, as Princess of Friendship. If her old library was good for the small village of Ponyville, the new library is graced with numerous and highly priced and valuable books, both great and small.
Pinkie Pie found joy in the value hidden within the still small library from the first day. She is regularly coming back, in search of books of direct interest, or randomly going through the books before her.
Pinkie Pie is found incapable of forgetting even the smallest detail, and having direct access to the wealth of knowledge she has been gathering: even if she had managed to forget her very own birthday, or other details of no humorous value.
As a Princess, Twilight Sparkle is holding on to books of value, books of great power in magic hidden among them. What happens, when the pink mare stumbles upon the dangerous material contained within these ancient tomes?
Ehh... dialog feels too forced and unnatural. That's something I struggle with too, so I'm not really sure how to suggest improvements