Bernie Sanders, the democratic socialist senator from Vermont, is a fierce critic of the affluent in America. That’s a bit rich, considering he himself is well off. Sanders, 77, has, in fact, amassed an estimated $2.5 million fortune from real estate, investments, government pensions—and earnings from three books, including the 2016 hit Our Revolution: A Future to Believe In. “I wrote a best-selling book. If you write a best-selling book, you can be a millionaire, too,” he recently told the New York Times, striking a downright Trumpian note. Since his bid for the Democratic nomination raised his profile in 2016,...