If you've ever read much about Science Fiction you will have seen the term "THE BIG THREE". The big three were writers who always came top of polls for the best science fiction writers. Their name were Isaac Asimov, Arthur C Clarke and Robert A Heinlein.
Firstly let me add these polls are pretty meaningless since they are nearly always American. Eastern European writers like Stanislaw Lem and the Strugatski Brothers are not that well read over here.
Certainly in America the writer who comes top is Heinlein, he is a very American writer. Now I have to add I am a fan of Asimov and Clarke but my relationship with Heinlein is more problematic.
Some of this is down to the fact that from about 1960 is fiction became very right wing and irratic. If you doubt this pick up I will Fear No Evil, The Number of the Beast (makes L Ron Hubbard look good!) and Time Enough for Love truly dire. He wrote the occasional good book (The Moon is a Harsh Mistress) but these are the exception rather than the rule.
It is often said that the 1959 publication of Starship Troopers marks this change but for all of its right wing politics it remains a page turning read. Indeed my advice to anyone embarking on Heinlein is to start pre 1960.
However Heinlein wrote some really good books/stories. I was reminded of this when Amazon "reminded" me that Baen books in America has published an omnibus of two of his short story collections The Green Hills of Earth (the title story is truly excellent) and the The Menace of Earth. These are part of his future histories series which was mostly published in the pulp magazines in the 40's and 50's (when Heinlein had moved on to the Saturday Evening Post). These were collected in a book call the Past Through Tomorrow which is one of those books I may never reread but I'll always keep!
Heinlein was also a fine writer of young adult science fiction and it can be argued that it is here he is at his best. Have Space Suit—Will Travel, Citizen of the Galaxy and Starman Jones rank amongst his best works.
Heinlein is not easy to get hold of these days (as are few of the great writers of the past). If you feel the need to try him then one of the juveniles mentioned about or any of his short story collections culled from the 1940's and 50's is worth picking up from a second hand bookshop.