(You want to buy Trixie's book don't you??)
We've got a book listing for a "Road Trip Special" next year that ties into an episode. Which episode exactly? That we don't know. It could be Road to Friendship, or it could be a future episode from season 9. With a July 2nd, 2019 release date, it's hard to say. The description is completely basic:
Let's hit the road! Join the ponies in this exciting leveled reader based on a special episode of My Little Pony: Friendship is Magic!
They have it set up to release as a Little Brown Book, so no huge chapter style tale in this one. We will get an update out once more information is released.
Thanks to Astral Phoenix for the heads up.