Dad books — you know, “serious” nonfiction titles about politics, current affairs, and the economy — are in a rough spot. Sales of nonfiction print titles are down nearly 8% this year, according to book tracker Circana BookScan.Partly to blame for the decline is a changing media diet, according to Pamela Paul, writer at large at the Wall Street Journal. “In the olden days … you got a James McPherson book about the Civil War. You've got David McCullough's biography of Truman,” said Paul. “Now you can listen to a podcast.”Paul joined “Marketplace” host Kai Ryssdal to discuss the decline of dad books. Click the audio player above to hear their conversation.