Warren Buffett, whose shrewd investing has swelled his Berkshire Hathaway assets to nearly $278 billion, reveals in a new biography that "the biggest mistake I ever made" was letting his late wife, Susie, walk out the door of their Omaha home.

Buffett, 78, who spent thousands of hours talking with author Alice Schroeder for her engrossing book The Snowball: Warren Buffett and the Business of Life admits he gave Susie plenty of reasons to leave.

One was Katharine Graham, publisher of The Washington Post and Newsweek. Graham, a 59-year-old widow, was smitten with the investment genius, then 46. When not trotting around the country "on business," they holed up at her Martha's Vineyard mansion. Rather than hide her flirtation, Graham would toss her house key to Buffett at parties.

Schroeder writes that Susie "made it plain to several friends that she was furious and humiliated," but sent Graham a letter granting her permission to date her husband. "Kay showed the letter to people as though it let her off the hook," writes Schroeder.

When Buffett was at home, he spent most of his time in his study. Telling one of her friends he was an "iceberg," Susie began a romance with her tennis coach, John McCabe. Then, in 1977, after falling in love with the arts scene in San Francisco, she informed Warren that she was buying a small apartment there. McCabe followed.

Meanwhile, Susie, expecting that Warren would fall apart without a woman to look after him, arranged for attractive blond Astrid Menks to cook and clean for him. She eventually moved into his house. Susie and Warren never divorced. After her death in 2004, he married Menks.