Dory Fantasmagory, Dory Dory Black Sheep Book 3 by Abby Hanlon

Ever since Dory met Rosabelle, a real true friend whose imagination and high spirits match her own, school has been pretty good. But now the class is learning to read, and it's proving to be a challenge for Dory. While Rosabelle can read chapter books in her head, Dory is stuck with baby books about a happy little farm. Dory wishes for a potion to turn her into a reader but things don't go as planned. Suddenly, a naughty little girl who looks an awful lot like Dory's imaginary nemesis, Mrs. Gobble Gracker, shows up. And a black sheep leaves the pages of the farm book to follow Dory to school. It really needs her help--this seems like a job for a superhero! And it would help if she knew how to read.