The Mongrel Rusty (part collie, part terrier), is a complication in the running of a Westchester County (N.Y.) kennel and canine hospital, until Sally, daughter of the house, persuades her parents to let her keep Rusty as a boarder until she proves an asset by way of precipitating the idea of a mongrel puppy business all its own. This new angle on the business helps pay off the mortgage, and the not too sentimentalized story of the combined efforts of the family and the dogs they raise conveys a warm sense of teamwork and accomplishment. The details of running the kennel are rather new in juvenile literature.