An idea book- that hardest of all types of books to promote. But a book that- in brief compass- takes issue with the foe of traditional American liberties. America, says Mr. Commager, has grown through diversity and experimentation, freedom of thought and expression. His concern is with the practical consequences to society of the current insistence on conformity. If in the name of security we hack away at our freedoms, in the end we'll forfeit that security we seek. We must maintain freedom not to indulge error, but to discover truth. The most important of the five essays that make up the book is the one on Guilt by Association, the most pernicious doctrine, he feels, that has ever found popular acceptance. ""It is pernicious in principle, in application, in consequences"" he says and proves that it is unsound, wrong legally, practically, historically and morally. An indictment that deserves to be read and read again.