Today's Hugh Downs concedes that emotional maturity is difficult if not impossible to define scientifically, but he plunges ahead nonetheless, for, after all, this is ""a purely informal presentation,"" an item which he admits falls into the category of ""self-help"" and features such tidbits of' advice as ""Always mix business and pleasure. It has been said, 'if you don't find pleasure in your work, you won't find pleasure'"" and quite a few eclectic quotes from the Upanishads, The Lonely Crowd, Joseph Fletcher of Situation Ethics, and Rene Dubos, among other lions who might be dragged into any jungle. The book isn't that bad. . . . It's just that it isn't that good. Best between commercials.