The GREATEST and Most POWERFUL pony has a book coming out in March of next year after many a delay and date change. Finally, we have a cover! Unfortunately it looks like they used the standard Trixie vector with a slight shift in the eyes. I bet G.M. Berrow will make up for that in words though!
The synopsis is still generic:
A new My Little Pony chapter book featuring your favorite unicorn pony and traveling magician, Trixie! An original story.
Thanks to Brony250 and JJ for the heads up!