Alex de Campi 
Agent Boo, Volume 1: The Littlest Agent [PDF ebook] 

With thousands of parallel worlds to keep in line, an Agent’s job is important, not to mention dangerous. But look out, Space City! Boo is about to become the Agents’ newest recruit! Sure, she’s small, and she does scare easily. But with a little luck, a lot of courage, and a grumpy cat named Pumpkin on her side, Agent Boo will learn that sometimes it’s good to be the littlest!