I just read in one of these darn books. I’ve totally lost track where I’ve read what. I think ”Remaking a 20th century legend”. I can’t recall the exact words but Simone told Sylvie the letters were all lost, but after her death, Sylvie found them neatly put away where she could not miss them when she was cleaning out her cupboards. They had already been edited by Simone so the authors insinuate she only wanted to make sure they were not published until after her death. It seems she knew Sylvie would publish them.
To add to the story, these authors think she may have left them as proof that she came up with some of Sartres ideas and since she had always denied that, she didn’t want it revealed until after her death.
I also read somewhere that some of Sartres wartime journals ( the notebooks) were lost. He sent them to Simone and she sent them for Bost to read during the war and Arlette thinks they may have been lost when Bost was injured in the war or just misplaced by Simone.

Everything you both say here sparks something else I read. In reading about Sartre and his existenialism, he says that the reason he choose writing as his lifes work was it was the only way he could find immortality, since he believed after death there was nothing. Simone had similar views.