Ho hum. Another book on the Catholic birth-control blow-up centering on Pope Paul's controversial Humanae Vitae. Mr. Derrick, an English writer, lecturer and critic, is a firm supporter of the pope's ban on chemical and mechanical birth-regulation, and his book is a commentary on and defense of the encyclical which promulgated that ban. His basic principle, like Pope Paul's, is stated honestly enough: ""Sex is babies."" The predictable arguments follow, beginning and ending (though not quite so honestly stated) with the ""natural law"" and its primary and secondary dictates and ornamented with the expected appeals to tradition and authority. The trouble with all this is twofold. First, from the academic standpoint, Mr. Derrick is arguing in a vacuum. Second, from the commercial stand-point, most Catholics are fair sick unto death of the controversy qua controversy; it is therefore difficult to imagine who will buy still another book on it -- particularly one where the author admits he advances ""an absurdly difficult and ascetic"" view.