“Saturday Night Live” turns fifty this year. Profiling its executive producer, Lorne Michaels, the New Yorker editor Susan Morrison sheds light on one of the most important people in show business. Morrison spent years talking to Michaels for her new book, “Lorne: The Man Who Invented Saturday Night Live,” and she includes recordings of those interviews in a conversation with David Remnick. “Lorne was a real student of what I call sort of the hinges between eras,” Morrison says. To keep the show current, Michaels “paid attention to replenishing the casts in a sort of seamless way, so that it would never seem like an old guy trying to do an entertainment for young people.” Plus, one of the show’s most notable alumni, Tina Fey—rumored to be a possible successor to Michaels, who is now eighty—reads an excerpt from the magazine’s review of the show’s first season, back in 1975.