Redwall

 Welcome to Redwall Abbey. Inside its enormous doors, mice live in peace, helping those in need and throwing epic feasts for the great and the good of Mossflower Woods. But outside a grave threat is gathering. An army of evil rats led by a vicious, one-eyed warlord, is on its way.
  Matthias is just one little mouse but he knows it'll take more than stones and mouse-sized arrows to keep the rats at bay. Enlisting the help of a military hare, wild sparrows and argumentative stoats, Matthias sets out to defend his freedom, his friends, and the abbey he calls home.

This is the first in a series of 22 books that take place in the Redwall world. My child tells me that it doesn't matter what order you read them in - just read the first one first.