My frustration,” writes Peter Schweizer in his new book, “Secret Empires: How the American Political Class Hides Corruption and Enriches Family and Friends,” “is not that the solid reporting on Trump has been too tough, but that the reporting on the Obama administration has been way too soft or in some cases nonexistent.” The author of the 2016 sensation “Clinton Cash” says Trump and his children didn’t invent the blurring of government and business, and details a number of ethical violations on both sides of the political aisle. One example: the little-noticed private equity firm run by the sons of...