Excepting the WWII memoirs (because I'm not sure how I want to count either Anne Frank, Elie Weisel or Winston Churchill in terms of "authors from the 1940s"), among authors whose nonfiction works are still read, there are a few names that do occur to me:
George Orwell, Friedrich Hayek, Simone de Beauvoir, John Maynard Keynes, Jean-Paul Sartre
For fiction there are obviously many more whose works have come through to frequent readership: Hemingway, Shirley Jackson, Albert Camus, C.S. Lewis, Antoine de Saint-Exupéry, Ayn Rand to name a few.
Lewis would be my example for nonfiction. Narnia is fun and all, but it's stuff like "The Inner Ring" (https://www.lewissociety.org/innerring/) where he really shines.
I agree (Planet Narnia also blew my mind), but I tried to make a judgement call over whether his non-fiction was "widely read" (I know very few people who have read any of it, relative to the number who have read the Narnia books).
Normally I wouldn't comment just to say the equivalent of +1, but I hadn't encountered "The Inner Ring" before, and it's great! Thanks for the recommendation.
George Orwell, Friedrich Hayek, Simone de Beauvoir, John Maynard Keynes, Jean-Paul Sartre
For fiction there are obviously many more whose works have come through to frequent readership: Hemingway, Shirley Jackson, Albert Camus, C.S. Lewis, Antoine de Saint-Exupéry, Ayn Rand to name a few.