I would be hard pressed to define classic Sci-fi Lit. any better than simply referencing Theodore (Ted) Sturgeon who was a Pulp Era Sci-fi writer and early television and film writer. Besides his pulp contributions, Sturgeon is probably best known as a writer for the Original Star Trek series. Sturgeon in Orbit is a collection of five stores published originally from 1951 to 1955 in Fantastic Adventure, Planet Stories, Fantastic, Startling and Other Worlds magazines.
These five tales which transcend most Pulp Sci-Fi, are more than adventure stories. Sturgeon explores the nature of humanity and the essential function of human relations through this collection. The first example, Extrapolation (Fantastic, 1953), begins as a tale of loneliness. Wolf Reger was a man so intelligent that he is almost prophetic. He "had so many talents that they were past enumerating." Reger could master science, math and engineering. He could predict actions and reactions, or extrapolate, for every invention he created. Reger's weakness was his relationship with humanity. In fact, their ignorance and lack of foresight would occasionally drive Reger into a rage. For this reason he could not work or live near his fellow humans. He lived alone until discovering a women who had attempted suicide who he nursed back to health and married. Reger's reformed attitude of humanity, based on his new romantic love, leads him to save humanity from an alien invasion. Isn't that the basis for most pulp era Sci-fi tales?
In the introduction to Sturgeon's novel Godbody, Robert Heinlein wrote that Sturgeon's message was that he loved us all. The same can be said of Sturgeon in Orbit as each of the stories is about humanity and science. I believe that few would argue that Ray Bradbury loved us too. Anyone who would write a novel in defense of artistic expression and preservation of knowledge and creativity like Fahrenheit 451 obviously cares.
I have just finished reading The Martian Chronicles which also is an expression of Bradbury's concern for humanity. As well as, his concern for unbridled bureaucracy and technology and the failure of the use of basic common sense. This concern is no less relevant sixty-five years after The Martian Chronicles began to be published.
When I begin to read the work's of Sturgeon or Bradbury, I am not seeking an investigation into the nature of human civilization but this is how these men approached their art. I would choose Fahrenheit 451 over The Martian Chronicles if reading for pleasure. I will add, about The Martian Chronicles, it is a bizarre book when the passages I enjoy most are an homage to The Fall of the House of Usher and a piece about the opening of the first Hot Dog Stand on Mars.