Invisible Inkling

The adventures of Brooklyn boy Hank Wolowitz and his invisible--but not imaginary--friend continue with The Whoopie Pie War, the third book in the Invisible Inkling series by Emily Jenkins.
A truck selling ice-cream whoopie pies sets up right in front of the ice-cream shop belonging to Hank's family, and it's taking away all the shop's business. His dad is going crazy. His mom is furious.
Hank and Inkling, his invisible bandapat, aren't going to take it. The Whoopie Pie War is on! They'll do whatever it takes to beat the whoopie pie truck--unicorn costumes, extreme kindness, an army of supervillains.

The illustrated chapter book's mix of silliness, fantasy, strong sense of place, and a realistic family make it a great pick for middle-grade readers.