"History doesn't repeat itself," Mark Twain observed, "but it often rhymes." Twain's saying seems prophetic when comparing the Gilded Age monopolies he satirized to today's Big Tech monopolies. Like the tycoons who built America's railroads, today's "digital railroad" companies like Google and Facebook that connect us online have generated unprecedented sums of wealth and power for just a handful of enterprises. Modern Robber Barons wear hoodies in Silicon Valley instead of top hats in the Rust Belt. Make no mistake. These underdressed tech titans are the new aristocrats, and their monopolies pervade our lives in ways that Rockefeller and Vanderbilt...