The Amazing Maurice and his Educated Rodents

This is in the Diskworld, but all new characters (with the exception of 2 and you will know them!). Maurice and a bunch of rats are surprised to find out that not only can they talk, but they are actually rather smart too. How did they get that way and what are they going to do about it is the substance of the story. They also get involved with the Mayor's daughter who inherited her mother's storymaker's ability.