The story of the American Society for the Prevention of Cruelty to Animals, as told in able manner by Lloyd Alexander, is full of humor and heart, as befits the subject. Here are tales of heroic rescues -- of as many as seventy-six horses aved at a clip--of boys who find their dogs and dogs their boys--of snakes in bathubs and wolves in midtown hotels. The ASPCA deals with 270,000 animals a year at a cost of $2,000,000; it handles 21,000 adoptions. It goes to court to stop cruelty whether to pets or to pawns of gambling rackets. It runs a Medical Stray Ward, an Animalport at Idlewild, numerous shelters, a Humane Work Division; it hands out information on pet care and obedience training. If the bustle here is on the heavyhanded side, it is portrayed with color and pep. For New York City dwellers there may be a particular appeal.