For whatever editorial reason, Campbell wanted any of Heinlein's stories that did not fit into this matrix to appear under an author's name other than Heinlein's. An author, if he wishes to continue being a paid author, always listens to his editor. So Heinlein supplied some other names for his other stories.
At one time Astounding took a readers' poll to see who their favorite Astounding authors were. The poll showed that Heinlein was the favorite, number two was Anson McDonald, and number three was Lyle Monroe. Both of those were pen names used by Heinlein!
Another bit of trivia: Heinlein's first published story was in the August, 1939 edition of Astounding. He was about thirty-two. In the very same issue was the first published story by Isaac Asimov, at that time a student at Columbia University and sixteen years old. The two worked together in The Navy Yard during WWII along with another author named L. Sprague De Camp, whom I'll mention a little later.
Okay, enough of that trivia. Another author of speculative fiction that ranks at or near the top is Ray Bradbury. Bradbury did not write the "hard" science fiction that the Big Three did—he often wrote stories that were hybrids of science fiction and fantasy. He also specialized in short stories much more than the others. Two novels, though, had movies made from them, the most well-known is Fahrenheit 451. One of best short stories, "All Summer in a Day," appeared in literature textbooks for junior high/high school. If you read his bio on Wikipedia you'll see that he was influenced by Heinlein and other excellent s.f. authors, but went his own way. If a teenager of either gender was to ask me what I would recommend for speculative fiction that was not "hard" science fiction, I would recommend Bradbury.
Fantasy fiction has several sub-genres, and one of the most successful is what has been called Sword and Sorcery. The man credited for creating this wonderfully entertaining genre is Robert E. Howard. He wrote primarily for the pulp magazine Weird Tales and it is there that the world first discovered Conan the Cimmerian and the Hyborian Age.
I've read that Howard claimed that as he wrote the Conan stories it felt as though some entity was there with him, looking over his shoulder and guiding his words. By the way, I've also read that publishers strongly advise you not put anything like that in a cover letter.
Robert Howard's fiction output was impressive, and he published poetry as well. As with the other authors, Wikipedia has a very thorough biography. Unfortunately he committed suicide at age thirty. Many years after his death all his unfinished writings and notes were passed from party to party until, by lucky chance, it came into the hands of L. Sprague De Camp. (I told you I'd get back to him.) De Camp plus two others, Bjorn Nyberg and Lin Carter, edited the finished stories, completed those not done and wrote complete stories from Howard's notes. However you felt about the two Conan movies starring Arnold Schwarzenegger, (I gave 'em a C-) the stories are much better and Conan is much smarter and agile than presented in those movies. Read them! And some of the other works by Howard as well, if you have any interest in that genre.
The next name on the list is one everybody will recognize: Stephen King. King may be the most successful fiction author ever if you include the movies made from his books and stories. And they are not all horror. The Shawshank Redemption, Lean on Me, and Total Recall are three movies I can recall without effort based on King's non-horror stories.
King is simply one hell of a writer. His use of similes particularly make me envious. I certainly have not read anything close to all his books—if I did that I wouldn't have time to write my own. Which reminds me, my current project, Saving Atlantis, was initially inspired by King's Bag of Bones, though you wouldn't know it if you read them both...I think. King's crowning achievement (pun intended) is his seven-book series, The Dark Tower. None of it is really horror, but it is definitely fantasy—mostly dark fantasy. Warning: Once you start that series it may be tough to stop. And some of those books are really long. My only objection to it is that volume 5, Wolves of the Calla wasn't nearly necessary enough to the overall story to justify the length. Only my un-asked-for opinion.
The number of high-quality speculative fiction writers is much too great for me to even try to guess it, and more are appearing every year. I have here mentioned five deceased authors because of their proven staying power. To the best of my knowledge everything published by the Big Three since 1939 has never been out of print.
Next time I'll thumb through a few other genres. I hope you'll be here. In the meantime, please, read and read and read some more.
One note slightly off subject: my monthly stories review is on the The Writers Table tab at Page and Spine. Good stories this month; you might want to check them out at www.pagespineficshowcase.com