The biggest book publisher in the United States is about to get bigger. ViacomCBS has agreed to sell Simon & Schuster to Penguin Random House for more than $2 billion in a deal that will create the first megapublisher. Penguin Random House, the largest book publisher in the United States, is owned by the German media conglomerate Bertelsmann. Adding Simon & Schuster, the third largest publisher, would create a book behemoth, a combination that could trigger antitrust concerns.