@AlastorHawkings I can't think of anyone who has never written *anything* outside their main "world". The best you can probably do is someone who's written the majority of their work within one universe.
First name that comes to mind is Raymond E. Feist. He's written more than twenty-five books in the Riftwar series, all taking place within that universe. I lost interest after twenty-three books, (partly because I became bored with fantasy in general) but I enjoyed the ones I read. Opening a new book in the series was always familiar - like reuniting with old friends.
The other author I really enjoyed, who has written a few books within one universe is James Barclay. Ten out of his twelve books are set on one world - The Raven books and the Elves books. I enjoyed all the Raven series, but I've never managed to read any of the Elves books. (yet)