Only on the "CBS This Morning" podcast, biographer James Kaplan shares why Irving Berlin has been called the greatest songwriter of American popular music. Talking with CBS News' Anne-Marie Green, Kaplan explains what was behind Berlin's nine decade career that included such songs like "God Bless America" and "White Christmas" that are still sung today. In his new book, "Irving Berlin: New York Genius," Kaplan shares how Berlin when from growing up as a Jewish immigrant in New York's Lower East Side and leaving home at the age of 13 after his father's death to writing over 1,500 songs.