Author Ben Mezrich normally writes about billionaires. Among his well-known titles are “The Accidental Billionaires: The Founding of Facebook, A Tale of Sex, Money, Genius, and Betrayal” and “Bringing Down the House: The Inside Story of Six MIT Students Who Took Vegas for Millions,” which were turned into the movies “The Social Network” and “21.”

In his newest book, “Charlie Numbers and the Woolly Mammoth,” Mezrich and his wife Tonya Mezrich pick a different kind of protagonist: a grade schooler who uncovers a prehistoric woolly mammoth tusk.

On Monday, the duo joined Boston Public Radio to discuss their new book, and how writing for adults differs from writing for children.