Too Many Pinkie Pies

Pinkie Pie was able to clone herself with the magic of the mirror pool in “Too Many Pinkie Pies.” Just how did she know about this place? More importantly, how did her Nana Pinkie know? Why did Twilight have a hidden book in the library in Ponyville detailing not only how it worked, but how to send the clones back?

Did Pinkie Pie ever return? Or are we now permanently in mirror world?