Subtitled ""The United Nations in Action,"" this is more than a guidebook to the structure and functioning of that ""goldfish bowl"" which, ""despite all its translucence... is still widely misunderstood."" It is also more than a collection of gossipy reminiscences by a man who for eight years served in a wide range of capacities within that unique international organization. Mr. Wadsowrth's book encompasses these approaches and a good deal besides. Preeminently the practical career diplomat, he holds no urgent brief for the UN as it now stands as the only possible salvation of the world. A patient, moderate man by experience as well as temperament, his view of his subject may best be described as evolutionary. The UN, he reminds us, ""is not and is not meant to be the last step in man's journey from the cave to the ideal state."" It is ""simply another step, and the best one yet,"" in a slow process that is still a long way from completion. He sees ""world rule through world law"" arriving some day, but ""only when mankind is ready for it."" Meanwhile there is a fascinating, if often frustrating, experiment going on in that direction, and he writes about his first-hand experiences in it with a sense of proportion, good humor and candor.