Franco's Spain is a country that, for all the recent tourism, remains very much a mystery to most Americans. Having been the New York Times Madrid correspondent from 1956 to 1962, Mr. Welles should and does have much of interest to say about it; for instance, that the Caudillo would win 65% of the vote if an election were held today, or that the post-Civil War Spaniard may want improvements in his situation but not at the price of violence. However, Mr. Welles' but-on-the-other-hand approach leaves the reader not very certain of anything. One of the least satisfying chapters, ""The Opposition,"" is a case in point. So is the title, the ambiguous irony of which may lead to a supposition that this is a superficial if intriguing exercise. Franco himself must be all but impenetrable to the foreigner, and so may the contemporary Spaniard's attitudes toward his recent past and near future, but considering the paucity of material on so fascinating a place and people, it is difficult to settle for what amounts to a bland political travelogue.