Bess Truman, like several other First Ladies, burned most of her private papers to stop them from being able to made public. But when the Truman home was cleared out in the 1980s (Bess Truman died in 1982 in case you don't remember), some of her letters (180, in fact) were found mixed in with other things - like being used as bookmarks in old books. These letters are going to be released by her grandson, Clifton Truman Daniel, in a couple of years, but he has provided a preview of these letters in an article in Prologue. This article isn't available online (unless you want to purchase it), but it is usually pretty easy to find a copy of Prologue if you are interested (I have a subscription myself).
In the bit on the website, they talk about the content of two of the letters - one that was written to Harry by Bess while he was in National Guard training camp in 1923 and one that was written in 1925 by Bess trying to convince Harry to let her bob her hair (she eventually won).
Margaret Truman Daniel used these letters when she wrote her biography of her mother, but did not release them, Clifton wrote that was probably in deference to her mother's wishes for privacy. She did allow 15 to be displayed in 1998 at the Truman Library, but Clifton, now the owner as Margaret Truman died last year, will release them in four years.
The letters span twenty years from 1923, when the Trumans were newlyweds to 1943, when Truman was a US Senator. These letters, when released, will be a huge asset to historians of the Truman family since they finally reveal something about the extremely private Bess Truman.