COLLECTED STORIES. By Tennessee Williams. New Directions. 574 pp. $19.95.

THIS BOOK is a posthumous monument to the brave career of the unknown Tennessee Williams -- the patient, undeviating prose writer who burrowed into mall private lives far removed from the calculated glitter of Broadway and Hollywood. It is true that a handful of his stories were the embryos for later plays and films such as Baby Doll, The Milk Train Doesn't Stop Here Anymore, The Night of the Iguana, even The Glass Menagerie and Cat on a Hot Tin Roof. But there are 50 tales in this volume, and they testify to almost a secret commitment, practiced every day, like the keeping of a highly imaginative and often merciless diary.

It is trite to say at this late date, but Tennessee Williams obviously wrote to survive. To quote his own telling line in one of the first-person stories, he practiced "a religion of endurance" by means of his work, and the further removed it was from the public hoopla that greeted his plays and films the more we can see the almost desperate nature of his literary need. Here are stories about bewildered, helpless, very human losers -- stories that are both unfalteringly understanding and pitiless, cutting far beneath the public image of the bright-eyed and ingratiating Tennessee that we recall from TV talk shows. Knowing these stories now in their entirety -- the climate of iron loneliness, the entrapment of desire, the futile beating of fists or hearts against what expressionlessly lies waiting -- we can easily understand why Williams often dissembled in public with the armoring of drink or drugs. Like his colleague Eugene O'Neill, he knew what the pits of hell were all about. But it's one thing to experience those pits and quite another to own the craft that will snare them in all their unholy glory. The astonishing thing about Williams the short story writer is that he found his style by the time he was 17 (that is no misprint) when he wrote the first story in this collection, entitled "The Vengeance of Nitocris" and published in Weird Tales in 1928. The narrative itself is full of sweeping theatrical effects that were ultimately to serve him better on the stage than the page, but what is uncanny is the simplicity and poise of the prose itself, which was never to change. Whether he was writing about ancient Egypt, as in this first story, or a Midwestern college town as in "The Field of Blue Children" (1939), he was always direct, clear, unaffected and unafraid to confront what most of us don't want to see.

ONE OVERPOWERING impression emerges from all these stories put together: Tennessee Williams (1911-83) knew more about the hidden life of far- flung America than any of us really suspected. He was a vagabond for many years before his smashing Broadway success in the 1940s, knowing working-class as well as bohemian environments from Mississippi all the way north to Provincetown, Massachusetts, spanning both coasts as well, and the fruit of all this knocking around dominates his early and perhaps freshest work. Here are salesmen, waiters, hobos, male and female hookers, slightly crazy young poets (using the bottled-up young Tennessee himself) and lonely young women who go to college or work in libraries. Few male writers can identify so convincingly with the less blunt sensibilities of the other sex -- the artistic revenge, if you will, of that "sissy" tag that was hung on Williams by his father, and caused him great pain until years later he came more or less to terms with both his father and his homosexuality.

One gets the feeling that unlike his peers in the American short story, Williams never consciously set out to write "masterpieces" as did, say, Katherine Anne Porter. Theater was his first public art while prose and poetry were almost his private compulsion, as necessary as breathing but hidden away from all the status-conscious pressures that besiege the craftsman who has everything riding on this form. Nevertheless Williams, especially in his pre-celebrity period, turned out stories that are the equal of any American writing done in his lifetime -- pieces like "Twenty-seven Wagons Full of Cotton," "The Accent of a Coming Foot," "Desire and the Black Masseur," "The Dark Room," and "Something About Him" have every chance of edging their way into permanence. The classic Williams story of this period has a purity of language and an unusual emotional depth, often with no reprieve, that simply humbles our standard responses. But as so often happens, Williams lost touch with his most basic materials after he became famous, and many of the stories from the late '50s to the end have that blowzy, international-set air that we can get from innumerable others.

One beauty, however, is "Two on a Party," written just before this last, sagging period set in. It tells the story of a burnt- out gay writer and an alcoholic nymphomaniac who cruise different U.S. cities together, each helping the other get what they need. In their own screwed-up, compulsive, dependent way, they are as much in love as any straight couple. Their fragile happiness, which we know can't last, is as good a symbol of what Williams stood for as any he ever created -- that every real person is a cripple of some kind, and the only true enemies are the self-appointed "whole people" who enforce a superficial and finally brutal version of reality on the more helpless.

Without a doubt, sadly, there are too many unnecessary stories in this handsomely produced book -- especially with the falling-off in the last two decades before Williams' death -- but it has the look and feel of a true labor of love, an artifact created in honor of an extraordinary individual. One day there will be a much tighter Selected Stories, but this original, grand gesture, in every sense, is the one to keep close.