This fetches up for me, memories from long ago. In my late teens, I think -- at any rate, a very long time ago now ! -- I happened upon one of Guareschi's Don Camillo books. I remember rather little about it: do recall that it revolved around a brief sojourn by the priest-hero, in the Soviet Union. I forget "how come"; but definitely have the impression that it was something low-key and un-alarming -- no perilous undercover stuff to bring succour to persecuted Christians ! As I remember, all was basically light-hearted -- as the books basically are -- with Don Camillo's chief sentiment about the place, being benign bemusement. I quite enjoyed the novel -- though without becoming an instant Don Camillo devotee.
The book, however, had a several pages' "afterword" by the author -- IIRC, without any connection with Don C's Russian doings: it was non-fictional and highly serious commentary on the -- in Guareschi's mind, utterly reprehensible -- directions in which he saw society, in his country and likely the world in general, going "at the time of writing". This outburst by the guy struck me as so angry, bitter and generally misanthropic: that my reaction was, "I want nothing more to do with this miserable bugger" -- I have not looked at anything by him, since.
After half a century or so, it occurs to me to wonder whether I was perhaps over-reacting: the novels themselves show for me, fair likelihood of being an enjoyable read -- that, irrespective of the author's "prophet-of-doom-hood". Realistically, I doubt whether -- too many higher priorities ! -- I shall in fact give them another try.