Bliven, after a long journalistic career which included thirty years of editing the New Republic, has composed what he describes as ""love letters"" to six great Americans: Benjamin Franklin, John Adams, Thomas Jefferson, Sojourner Truth, Emerson and Thoreau. Unfortunately as often happens with billets-doux, there is more gush than bite and judgments are sometimes askew. For example Bliven interprets Emerson's ""Hell is better than Heaven, if a man in Hell knows his place"" as proof that Emerson ""never made a pretense of being what he was not"" -- even when out of context? The portraits of the other eminences are packed with clusters of accomplishments, utterances, marginalia -- a hurried skim, with these few odd simplifications: ""it was [Jefferson] who insisted upon. . . the Bill of Rights""; or Franklin ""discovered the Gulf Stream."" Obviously not a scholarly work but it probably will achieve some currency since four of these monographs have appeared in the Reader's Digest and they may benefit slightly from a bicentennial push.