The billionaire acquired bookstore chain Barnes & Noble, which had suffered seven years of declining revenue, for $683 million in 2019 and installed a new CEO who fired nearly half of the company’s New York-based book buyers and allowed store managers to decide which books to buy. Singer, who lives in New York City, took a $3.2 billion stake in telecom giant AT&T and called for sweeping changes in the underperforming company. Less than a year later, the chief executive stepped down and his successor announced plans to undo a huge acquisition by spinning off WarnerMedia.