The discovery described by museum staff as "exceptional" was made during conservation work on shoes that make up part of the permanent exhibition at the Auschwitz museum.
The release of the letters marks the 80th anniversary of the first transport of Polish prisoners to Auschwitz.
Fifty thousand people visited Poland's Auschwitz Museum in March thanks to virtual online visits, the former Nazi German death camp's authorities announced on Twitter on Thursday.
The unique collection sent to WWII Polish diplomats known as the Ładoś Group who tried to save the Jews from being murdered, was acquired from a private owner in Israel thanks to the efforts of the Polish Embassy in Bern and Markus Blechner, a Polish honorary consul in Zurich.
Nancy Pelosi, the Speaker of the United States House of Representatives, visited on Tuesday the site of the former Nazi German Auschwitz death camp in southern Poland. Pelosi was accompanied by Polish Sejm (lower house) and Senate speakers.
An online lesson presenting the fate of Jews held in the Litzmannstadt Ghetto, set up in the Nazi German-occupied central Polish city of Łódź during World War II, was posted on the Auschwitz Museum's website on Monday.
Agnieszka Sieradzka of the museum’s art collection department said the works present life in the camp, punishments, deaths during escape attempts, marches of death and recollections and portraits of people the artist remembered.
The main international events to mark the 75th anniversary of liberation of the Nazi German death camp Auschwitz-Birkenau will be held on January 27, 2020 on the site of the former camp, Polish Ambassador to Israel Marek Magierowski has said.
The fate of Greek Jews deported by Germans to the Auschwitz Nazi German death camp during WWII is presented through the stories of concrete people at a new temporary exhibition at the Memorial and Museum Auschwitz-Birkenau in southern Poland.
Polish President Andrzej Duda and US Vice-President Mike Pence are to visit the former Nazi-German death camp of Auschwitz in southern Poland on Friday, a symbol of Jews' extermination and the martyrdom of Poles and other nations.