In 1996 Nicholas Sparks’ romantic novel “The Notebook” became a bestseller. Eight years later it was adapted to a classic movie starring Ryan Gosling and Rachel McAdams. And the love story that was “a song, a dream, a whisper,” is now a Broadway musical. Correspondent David Pogue talks with Sparks and with members of the creative team that turned his book into a show that is already causing considerable audience joy and weeping.