From the back cover:
"Have you ever wishes a wish? Do you remember what you wished for? Wait, don't tell me, because then your wish might not come true. I never dreamed wishes could come true. At least not until my totally true imaginary friend Posey stumbled on a real wishing well and cast a special wishing-well spell. And guess what? Almost everyone in town had cast wishes down that well. Sounds great, right? Well, maybe not. Let's just say be careful what you wish for, because with Posey around, it might just come true."