It’s a side of Jacqueline Kennedy only friends and family knew. Funny and inquisitive, canny and cutting. In “Jacqueline Kennedy: Historic Conversations on Life With John F. Kennedy,” the former first lady was not yet the jet-setting celebrity of the late 1960s or the literary editor of the 1970s and 1980s. But she was also nothing like the soft-spoken fashion icon of the three previous years. She was in her mid-30s, recently widowed, but dry-eyed and determined to set down her thoughts for history.