The rats are coming!
Welcome to the collected volumes of The Adventures of Susan and Tucker!

Ten-year-old Susan's parents dragged her away from her friends to spend a wet and dreary summer in a new place. Unhappy with her lonely, boring life, she adopts Tucker, a butterscotch tabby, thinking he is just an ordinary cat. But when he rescues her from the rats and brings her to his magical city deep underground, Susan falls headfirst into a war between the enchanted cats who live under her house and the evil rats who want to claim Cat City for their very own.

Including all three titles in the series:
Cat City
The Rat King
The Be-All and End-All