The End of All Halloweens - The Adventures of Übergirl, #3

Nine-year-old Kelly Donovan helped save the world from aliens, but now she has an even bigger problem-someone's trying to ruin Halloween. Life is difficult for the world's tiniest superhero. Her genius plan to save her parents from supervillainy in one easy step proved impossible. at least not without destroying the Earth. Superpowers, good or bad, aren't going away any time soon. Mom's as likely to hug her as trick her into a ridiculous trap, but at least her dad's mellowed out a bit.
Lately, he seems focused more on being weird than evil. Small favors. Kelly's looking forward to the happy normal of trick-or-treating. until candy starts disappearing. Convinced a mysterious supervillain is responsible, she and her best-only-friend Paige rush to stop all the sweets in the world from evaporating. Alas, with only a week left and zero clues, humanity will likely suffer an even worse fate than enslavement by an alien race: the end of all Halloweens.