Mouseheart by Lisa Fiedler (author)

Hopper is just an ordinary pet shop mouse before he escapes. Soon he finds himself below the bustling streets of Brooklyn, deep within the web of transit tunnels, and in Atlantia, a glorious utopian rat civilization.

But all is not what it seems. Firstly, there are cats everywherein Atlantia, cats that would normally eat a rodent before you could blink, yet strangely leave the citizens unharmed… and no one can answer why. And although Hopper is treated as a royal guest, he misses his siblings that were lost in the escape attempt and is desperate to find them. But Atlantia is constantly threatened by rebels who wish to bring the city to its knees. Soon, Hopper is caught in the crosshairs of an epic battle, one that crosses generations and species.

As the clashes rage, Hopper learns terrible, extraordinary secrets: Deadly secrets about Atlantia. Painful secrets about his friends. And one powerful secret about his destiny…