The Dealer Who Dared : AN ARTFUL LIFE; A Biography of D. H. Kahnweiler, 1884-1979 <i> By Pierre Assouline (Grove Weidenfeld: $21.95; 416 pp.) </i>

When Daniel Henry Kahnweiler opened his first gallery in a tiny storefront on the rue Vignon in Paris in 1907, the artists he championed (among them Picasso and Braque) were living marginal lives, and the only people who thought to buy modern art were a few idiosyncratic collectors.

By the time Kahnweiler died in 1979 at age 94, his gallery (which by then bore the name of his sister-in-law, Louise Leiris) was listed among the top 500 export firms in France, and the daily paperwork--with galleries and museums around the world--�had become unbearable.�

Pierre Assouline’s new biography--the French title, L’homme de l’art, has been infelicitously translated as “An Artful Life�--shows how Kahnweiler guided the careers of Picasso, Braque, Derain, Leger, Gris and others through this 20th-Century sea change in taste and consciousness. The artists with whom Kahnweiler was associated for the greater part of his life comprise what is nowadays, almost as a matter of course, called the “heroic period� of modern art--Assouline titles one chapter “The Heroic Years�--but the dealer whom Assouline describes doesn’t exactly fit the heroic mold: He’s reserved and methodical, albeit with nerves of steel. This German Jew who spent most of his life in Paris makes a somewhat elusive subject for a biography; Kahnweiler is a legendary behind-the-scenes player in the drama of modern art.

Kahnweiler had a visionary grasp of the new Cubist painting. Visiting Picasso’s studio in 1907, he found the unfinished “Demoiselles d’Avignon�--which, with its startling distortions, made many artists uneasy--�indescribable . . . indefinable . . . admirable.�

Equally important, he had a visionary business sense. He insisted, whenever possible, that his artists have exclusive relationships with his gallery. The artist gave Kahnweiler much or most of his output in exchange for fixed prices per painting and, sometimes, a monthly stipend.

Coming onto the scene at a time when the most challenging art inspired no interest in the museums and little interest in the establishment galleries and periodicals, Kahnweiler saw himself as the artists’ advocate: He felt confident that he would manage the difficult struggle to find acceptance for this art that was turning the order of nature inside out. He was one of the first dealers to systematically photograph works by his artists, to attend to their personal affairs (helping to look for apartments, for example).

After World War II, when Kahnweiler became Picasso’s exclusive agent, an idea of the dealer that was half a century in the making had become big business.

Yet Kahnweiler’s path was by no means straight. As a German and a Jew, he could not avoid the trials of two world wars. He spent the years of World War I in Switzerland, in enforced semi-retirement. He did try to keep up his work through correspondence, but it was painful to watch as his artists (understandably) made arrangements with other dealers, and, worse yet, his stock of paintings was seized by the French government as the property of a German national.

When Kahnweiler returned to France between world wars, he launched a campaign to regain his incomparable stock of Cubist paintings, or at least prevent its sale at auction, which he feared would flood the market. Many of his artists had fought for France, and the veterans Braque, Derain, and Leger gave support to Kahnweiler’s campaign. But anti-German feeling ran very high; the efforts were to no avail, and in a cycle of sales at the Hotel Drouot, Kahnweiler found the fruits of his heroic years sold off to the highest bidders amid a circus atmosphere (and the bids were very low).

The Drouot sales, along with a new, speculative and even explosive market for modern art, seem to have turned Kahnweiler into something of an outsider in the Paris of the ‘20s. (Assouline calls his section on these years “Crossing the Desert.�) But at least when World War II came, Kahnweiler was better prepared to weather another storm.

He spent the World War II years in seclusion, sometimes in hiding in the south of France, while his sister-in-law Louise, French and not Jewish, took over the gallery. The stock was safe, and after the war, Kahnweiler could return to take his place as a grand old man presiding over the postwar explosion of interest in modern art.

The enforced semi-retirements of the war years did afford Kahnweiler an opportunity to write. During World War I, he set down his classic account of “The Rise of Cubism�; during World War II, he wrote a biography of Juan Gris, whose untimely death in 1927 at age 40 had affected him deeply. One book that Kahnweiler never got around to writing was his memoirs (although a series of radio interviews was published as “My Galleries and Artists�).

In later years, Kahnweiler’s inability to compose his memoirs became something of a frustration, but I do not think it strange that the discreet Kahnweiler held off from the task. The flamboyance of the autobiographical “I� was not for this man who had been a quiet friend and confidant of the outrageously flamboyant Picasso.

Kahnweiler held culture and commerce in a classical balance. He matched the boldness of the new art with the boldness of his business sense; he matched the severe grisaille of the Cubist painters with the discipline of the dealer who sat, day in and day out, waiting for the customers to arrive. And as that strange Cubist universe came to look beautiful, the customers did come.

Anyone who has visited the Galerie Louise Leiris in its present location on the rue de Monceau in Paris has experienced the Kahnweiler ideal: The white walls and simple wood frames allow the glorious works by Braque, Leger and Picasso to overwhelm us quietly, almost casually.

In the terrible days just after World War I, when Kahnweiler’s fortunes had sunk so low, Leger sent him the definitive homage: “You were the first person who dared, and we know that your name will always be fixed in the annals of modern art.�