I mów, że moja chwała z przyjaciół się bierze

This was a really insightful read! As a disclaimer, this is probably the first book I read which is basically a letter correspondence.
Here, we are able to get a little bit into the heads of two of the titans of XX century (science) fiction authors (the parenthesis is intentional, as it is not really what they considered their work to be). It's genuinely eye opening to dive into their thoughts on the sci-fi scene, each other's works, trends in literature and some politics. Somehow mind-blowing to me was the fact, that Lem did not speak English and only learnt to write and read in it. Especially considering the quality of English in his letters.
However, I think this book is mostly interesting to people who know the works of both of the authors. Otherwise, it can be difficult to relate to what are they talking about.