Some of the writers who produce books and articles about music are trying to help people form opinions. Others offer their own. Both approaches are useful. There is a cousin to these critical siblings: writing that claims to help shape opinion but dispenses more than it nurtures. That is all right, too, but it sometimes wants careful reading.
Two parties of the third kind have recently been heard from: The paperback edition of ''All American Music,'' by John Rockwell, a critic with the New York Times, has come out from Random House's Vantage Books; and publisher David Godine of Boston has just released Samuel Lipman's second book of essays, ''The House of Music - Art in an Era of Institutions.'' Mr. Lipman is a concert pianist, a teacher at Aspen, and, among other things, a music critic with the magazine Commentary and publisher of The New Criterion, a magazine of art criticism. Both critics live in New York, but, as it were, at very opposite ends of town.
What they are about, together, is essentially the Musical Left and the Musical Right: that is, as far as the gamut they personify, of present-day commentators on the musical scene.
In the far left corner we find Mr. Rockwell, holding forth effusively in the New York Times on what he sees as the coming together of all the important musics of our day. The dream that inspires him is the eventual mainstreaming and homogenizing of everything from the most intellectualized concert music (Elliott Carter, for example) to the most Neanderthal of the punk rock groups (Talking Heads).
In between, as we gather from ''All American Music,'' Rockwell finds (or knows) very little outside the subjects of his category-essays that is worth talking about, and the randomness of his choice(s) impresses one more than the soundness of his polemic.
Rockwell engages to a degree in what I call musical Maoism. He weighs the various musical types and approaches in the same balance, treating them all as having equal weight and equal potency for being punted between (and straddling) the many sectors of our pluralistic culture. He may have his finger on the crossover phenomenon we've seen the communications media help bring about, but he is musically unqualified for drawing any conclusions about who is likely to be nourished by it all - and for how long. He confuses cultural progress with the deterioration of both professional and lay musical standards in America, and with mass-market ''success'' in an art product.
All this is taken up coolly by Samuel Lipman in ''The House of Music.'' Unfortunately, he does so with such disdain that one gets the sense, after all the Jeremiac disparagement he righteously flings around, that there is very little left in music that he really cares for, let alone loves. Lipman, like a defender of the faith, convincingly inveighs against numerous shabbinesses about today's musical scene, such as the artistically bankrupt Metropolitan Opera (no commitment to contemporary opera in 50 years). But I fear his constant verging on misanthropy is offset only by his holifying (and hoarding, one might say) what is good about the scene. Astute and well-intentioned as he may be, his almost crippling elitism tends to leave everyone who is not Samuel Lipman (a good many of us, by last count) rather out in the cold, as far as what is left in the art to warm, encourage, and gladden us.
Both Lipman and Rockwell are very readable. But it is hard to imagine that music-loving readers - who turn to critics for help in making informed judgments - are well served either by doomsaying Cassandras who all but forecast the end of music itself, or by dogged inclusionists who see it all continuing as one ''mellow,'' unilevel happening.
Where the art is, is where the head and the heart commingle. That's where the joy is, too.