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[Sebatianos]

Well Sillmarilion was his crown work - but he never published it.
It was the story he started writing still during world war 1. It was a complete story that tried to explain how the world came to be, how creatures were created, why different races behave the way they do...
Actually it was more of a pseudo-religious text then anything else. The other works were simply side-stories to this work.
Of course Silmarillion never got that popular - it's not so much into heroic tales - it's more a history book of the Middle Earth.
After J.R.R. died his son published Silmarillion - but he also had to edit it and make a sort of conclusion. Tolkien himself never felt the book was completely ready for the public!