This is cool! I just found this article, made available by the Mises Institute, from a 2007 volume of the Journal of Libertarian Studies: Mises and Rothbard Letters to Ayn Rand. Both letters were written following publication of Atlas Shrugged.
If you’re unfamiliar with Mises or Rothbard, start with their Wikipedias. I’m currently plugging through the first pages of Mises’s Human Action. I might finish in a year. Rothbard’s Man, Economy, and State is next. I’ve read some Rothbard before, and he’s among my favorite authors. So far, his ideas align with mine better than anyone I’ve studied, including Rand. So it’s fascinating to see in this letter his adulation for her and the profound impact of Atlas Shrugged, similar to what it did for me.