Her reticence was because of, in part, her father's suicide when she was 18, Daniel writes in the fall 2009 issue of Prologue magazine.
"She adored him, and his death was sudden, unexpected, heartbreaking, and in that day and age, shameful," he wrote. "She never spoke of him. What thoughts and feelings she had were reserved for family and close friends, and she was determined to keep it that way."
But, Daniel said, his grandmother overlooked 180 letters that had been hidden away in books, or in the back of desk drawers. They were discovered in the early 1980s by Truman library archivists.
Daniel's mother Margaret, who owned the letters until her death last year, included some of them in her 1985 biography of her mother. In 1998, 15 were put on display at the library. Daniel is working on a book of his own, on his grandmother's letters, but it will be another four years before all of them will be open to the public.
Daniel said although, as a child, he knew his grandmother as a stern lady who he feared disobeying, her letters showed him the softer, playful side he had heard stories about while growing up.
"I just started laughing when I was reading them for the first time," he said. "It was fun to finally confirm -- she has a sense of humor, she's fun to listen to, and also to listen on paper to a young Bess Truman. I only knew the 80-year-old Bess Truman, I didn't know the 40-year-old Bess Truman."
Daniel said his favorite letter is the one in which Bess recalls killing a bug in her bed.
"I am the bug killer in my house," he said, laughing. "And if my wife is alone without me, then she'll have to do it on her own."
While he and his wife don't write letters to each other, they do text message.
"My grandparents' letters are the 1920 equivalent of texting or e-mailing," he said.
This story has been updated to reflect Mary Margaret Truman as "Margaret".