Last September, Daniel Wildenstein’s family celebrated his 80th birthday with a party at the two-star restaurant Laurent, one of Paris’s most fashionable spots. It was an intimate dinner, for about 70 guests, to which Daniel’s 64-year-old second wife, Sylvia, had invited only her husband’s closest friends and his family, whose members had flown in for the occasion from New York, Montreal, and Palm Beach, some of them on the family’s private Gulfstream IV. Daniel’s two sons, Alec, 57, and Guy, 52, and Guy’s wife, Kristina, were there, as were all six of his grandchildren. Most of the friends in attendance that night were from the horse-racing world, trainers and jockeys who had worked with the Wildensteins over the years. Horse racing was the old man’s passion, and his stable, Allez France, is considered among the best in Europe.

Almost no one from the international art world was there, even though there were few people in that world whom Daniel Wildenstein, probably the richest and most powerful art dealer on earth, didn’t know. Wildenstein, always aloof from his peers, rarely ventured out in recent years, except for regular excursions to the track and his office.

As friends and family toasted him that night, Daniel appeared relaxed and happy. It was an evening devoted to celebration, and there was a lot to celebrate. So many things had turned out as he had planned. In the three and a half decades since he had taken over the 123-year-old family business from his father, Georges, Daniel had added greatly to the luster of the Wildenstein name and also amassed enormous wealth; estimated at more than $5 billion, his fortune was the only one of that magnitude ever made in the art market. People didn’t trifle with him or with his family, over which he maintained an iron control.

They said behind his back that he was “mean,” “ruthless,” and “frightening,” but few really knew him, because that was how he wanted it. Other families had...