BOOK REVIEW : Keeping an Eye On the ‘Thought Police’ : FREE SPEECH FOR ME--BUT NOT FOR THEE: How the American Left and Right Relentlessly Censor Each Other <i> by Nat Hentoff</i> HarperCollins/Harry Asher Books. $25; 405 pages

In 1885, the public library of Concord, Mass., condemned the newly published “Huckleberry Finn” as “the veriest trash” and banned the book from its shelves. “If Mr. Clemens cannot think of something better to tell our pure-minded lads and lasses,” said an outraged Louisa May Alcott, “he had best stop writing for them.”

Not much has changed in the last century or so, as Nat Hentoff reminds us in “Free Speech for Me--But Not for Thee.”

Bowdlerizers, blue-noses and bullies on both the left and the right are still seeking to ban “Huckleberry Finn” and countless other modes of expression ranging from soapbox oratory to stand-up comedy, from Playboy pin-ups to political broadsheets.

“Censorship--throughout this sweet land of liberty--remains the strongest drive in human nature, with sex a weak second,” writes Hentoff. “Only a small percentage of these assaults on speech and assembly are covered by the press, for it has only so much space. These small silencings in places remote from the big cities are known only to the natives and the victims.”

“Free Speech for Me” is Hentoff’s effort to vindicate the much-abused First Amendment by bringing the “small silencings” to public attention.

What makes the book especially provocative is Hentoff’s insistent focus on acts of censorship by those who ought to know better, including librarians, teachers, journalists and university administrators.

Almost at random, Hentoff surveys a parade of First Amendment horribles: the attempt by the staff of the “Village Voice” to kill a cartoon strip by Jules Feiffer because of its ironic reference to the word nigger , the punishment imposed by Yale University on a student for circulating a gay-baiting poster, and the cancellation of a performance by a Turkish folk dance troupe in San Francisco by then-mayor Dianne Feinstein out of fear of Armenian protests.

Some of the controversies, such as the efforts to ban a demonstration by self-styled Nazis in Skokie, are already notorious.

Others are more subtle and obscure--Hentoff, for instance, tells the story of two fundamentalist Christian students who were banished from a Pennsylvania classroom and given a failing grade when they objected to reading Studs Terkel’s “Working” because of its profane language and its references to prostitution.

“Once again, I marvel at the concentrated zeal with which all these grown-ups were determined to force feed these primitives,” Hentoff writes. “Why couldn’t they have given the boys another book?”

To his credit, Hentoff is never guilty of a knee-jerk response, except perhaps in the sense that he is a purist on the subject of the First Amendment and invariably ends up on the side of more freedom of expression.

He has probed deeply into the inner workings of censorship, he has weighed its real-word impact on our lives, and he points out some of the unintended and ironic results of well-intentioned acts of censorship.

Houston, for example, once enacted an ordinance that prevented the municipal library from acquiring books from any company “in contact” with South Africa.

As a result, the library was no longer able to buy a directory published by a black Methodist church because of its dealings with South Africa: “That contact,” explains the Houston librarian, “consists of providing food and clothing to people in need there.”

Hentoff is a scribe-of-all-trades who has distinguished himself as a jazz critic, novelist, biographer, and author of children’s books, some of which have themselves been banned. Indeed, Hentoff himself was once excommunicated by a group of rabbis who objected to his criticism of the Israeli invasion of Lebanon in 1982.

Above all, he delights in his role as a kind of self-appointed First Amendment ombudsman, and “Free Speech for Me” may be regarded as a summing-up of several decades of journalistic activism. Indeed, because “Free Speech for Me” appears to be drawn from Hentoff’s work as a syndicated newspaper columnist, I thought I detected the tool marks of a cut-and-paste job--too many newspaper-style one-sentence paragraphs, too little effort to explain the loftier principles of free speech in a democracy.

But Hentoff is one true believer who utterly refuses to preach, and he is less interested in the theory than the practice of free speech--”Free Speech for Me” is truly a user’s manual for the First Amendment. And one cannot help but feel invigorated by the sheer heat of Hentoff’s righteous indignation, grateful for his courage and vigilance, and awe-struck by his tireless resistance to what he calls “the thought police.”