THERE have probably been few if any previous elections in which the “sincerity” of the presidential candidates was as much of art issue as in 1972. In the middle of this political morality play, in October, the Harvard University Press published Lionel Trilling’s new book, Sincerity and Authenticity. This is not to suggest that there is an immediate connection between the turbulence of the political scene and the (one presumes) tranquil refuges in which men of letters write books. (In any case, the present book contains lectures given by Trilling in the spring of 1970.) There are less immediate connections, though, and it is worth uncovering them for a better understanding of letters as well as politics.