Since the open-pore Sixties, increasing numbers of young and venturesome Americans have succumbed to the pleasures of the dry-heat bath considered in Finland to be ""holy as any church. . . therapeutic as any hospital."" Here, minus the mystique, is what anyone who'd like to join their ranks needs to know--following a capsule history of steam baths, strong on Native American usage, for thorough-going adherents. You'll learn how to take a sauna (not too seriously: temperature and humidity, for instance, are a matter of taste); the physiological effects, medical cautions (for heart patients especially), and side benefits (weight loss is not one; what drops off is water); and--at considerable length--how to build a sauna or buy ready-made components. All presented in an easygoing, direct manner, with marginal curiosa (""An Englishman Takes a Sauna,"" ""Sauna and Your Long Hair"") as well as drawings of nude bathers that won't prove an embarrassment.