Those old buddies, big-time operator Harry the Cat and scrounger Tucker Mouse, are keeping house again in their Times Square drainpipe, and have grown domestic enough to become sentimental over a homeless dishrag of a pup whom they name Huppy. As Tucker himself is the first to admit, there still seems something ludicrous about a cat and a mouse making such good room-mates, but Huppy's pet-as-surrogate-child role is easy enough to fathom. And after the rapidly growing dog wreaks babyish havoc in Tucker's neat house, the problem of finding him a new home is complicated by parent-like fears that Huppy will be corrupted by a gang of street dogs. Nevertheless, plans to send Huppy to Connecticut (where Harry and Tucker have. . . Countryside connections, remember?) are rejected because this is a ""new York dog with New York problems,"" and indomitable Harry begins courting high-toned, Siamese blooded Miss Catherine who might be induced to share her cushy uptown apartment with a deserving pup. Selden pokes gentle fun at the animals' New York accents and at their behavior -- half grown-up wise guy, half squabbling kid. But most of the fun still lies in their elaborate attempts to live up to human standards, with the details of Tucker's aspirations to drainpipe elegance the most fascinating. Big as he is, Huppy is a more manageable problem than the two tackled in their last, rural adventure, and Garth Williams shows just how every improbable encounter actually looked. It's good to see the old guys back home again where they belong.