A file-emptying defense of the F.B.I's procedures by the busy Overstreets who have given considerable attention in the last decade to Communism and the Left, here and abroad. Beginning with a review of the tangled beginnings of the agency, under the leadership of that grand-nephew of Napoleon, Attorney, General Bonaparte, and a distressed view of the ""dark years"" of vigilantes and dragnet methods in the Thirties, the authors then define the evolution of the F.B.I's present jurisdiction: ""to enforce laws passed by Congress and to perform its domestic intelligence function without infringing on civil and constitutional rights. . ."" The Overstreets' further explication wanders on to the muddy ground where the battle is truly joined: ""and to respect and uphold these rights without letting the exercise of them tear apart the structure of liberty under law."" Exactly when is the exercise of guaranteed rights a danger to the nation? Although the Overstreets firmly parallel the ""threat"" of the Right to that of the Left, liberal critics will hardly be comforted by the authors' naming as ""vital targets"" of Communist propaganda ""young people, intellectuals, minority groups, peace groups and others."" Too much foggy inference detracts from what could have been a clarification of the on going discussion of ""present dangers."" The bulk of the book is concerned with peppered rebuttals -- some irrefutable -- of current and past critics of the F.B.I. However, the treatise as a whole is more argumentative than scholarly or illuminating.