In 2008 — the same year that Robert Downey Jr. appeared in the action comedy “Tropic Thunder,” for which he would earn his second Oscar nomination — he also appeared as the billionaire inventor and unlikely superhero Tony Stark in “Iron Man,” the debut feature from the upstart Marvel Studios.Downey lost the Oscar (to Heath Ledger in “The Dark Knight”), but Marvel won the day. In the 15 years since “Iron Man” came out, the Marvel Cinematic Universe has expanded to 32 films that have earned a staggering $26 billion and changed the world of moviemaking for a generation. In a new book, “MCU: The Reign of Marvel Studios,” the writers Joanna Robinson, Dave Gonzales and Gavin Edwards explore the company’s scrappy beginnings, phenomenal success and uncertain hold on the future, with lots of dish along the way.On this week’s episode, Gonzales and Robinson join the host Gilbert Cruz to talk all things Marvel.