Talk:Chapter books/@comment-5288784-20171228213208

"At least one of the MLP books that Berrow wrote has relationship to the fourth season of Friendship is Magic."

Considering the date of the tweet, I believe she was referring to the Ponypalooza book, which hinted at Pinkie Pie having a third sister, which in the fourth season turned out to be Maud Pie.

The next possibility in line (assuming she already had that one planned at that point) is the Rainbow Dash's one, which has obvious references to the episode Daring Don't.