This is a classic for a reason. One of the best written children's animal stories around, the characters and descriptions are excellent and the plot is compelling and innovative. Rats aren't usually considered heroic creatures, yet no one will fail to identify with the characters. The book is not stereotyped or predictable and is immensly readable. The story [and the book in general] is more complex and compelling than it first appears.
Two sequels were written, but they were written by the author's daughter and unfortunately do not live up to the same standards.