HELEN MACINNES showed in Above Suspicion that she can write a superior thriller, and Assignment in Brittany is another proof that she can. Although it has the ingredients of a thriller — espionage, Gestapo, commandos, man hunt, death in an inn and in quicksands, underground hiding-places it has an unusually interesting theme: the attempted assumption of one man’s personality by another. The style is always restrained and sober, the incidents without melodramatic forcing. One easily swallows the initial improbability, of an Englishman impersonating a Frenchman, for the sake of the story. After that, the descriptions of Breton towns, countryside, and types strike one as records of actual observation, and the suspense never flags. Hearane, the hero, is a properly modest adventurer, and the man he impersonates an interesting degenerate. The gradual recognition by the Corlay family of the deception is plausibly handled. A really good adventure story should deal with people who are more than pawns pushed about in an imbroglio, and the characters here are all that. Madame Corlay, Anne. Elise, Albertine, Myles, Henri, with Hearne and the absent Bertrand, as well as the many Breton fishermen and German soldiers and agents, all have the air of truth, as has the grim, brooding tension of the war in the background. Especially clever is the way in which the “assignment,”which had seemed so completely prepared for in London, is complicated by human intangibles in Deodat. R.M.G.