On a fateful night in April 2019, John Paul Mac Isaac was working at his store, The Mac Shop in Wilmington, Del., when a customer walked in who would upend his life. In his forthcoming book, “American Injustice: My Battle to Expose the Truth,” Isaac will tell his story — and here, in an exclusive first look, he describes the moment he met Hunter Biden. It was a Friday night, 10 minutes before the shop’s closing time. I was checking out a website about CNC machines and woodworking. I had no intention of working late; I was ready to go...