Pirates of Underwhere by Bruce Hale

Stephanie has a whole underwear drawer full of trouble.

A week ago, Stephanie's biggest problem was finding enough time to complete her homework and study for her Mathletes competition. Now, thanks to her big-mouth brother, Zeke, she has to deal with magical toilet brushes, sinister talking cats, nearsighted sea serpents, singing custodians, feminist pirates, runty freedom fighters, and all the cottony-white weirdness of Underwhere—the world beneath our own where people wear their undies on the outside of their clothes.