J.R.R. Tolkien

Most likely to be the most admired fantasy author.
He has written quite a few books, but they which has been written describe a most detailed imaginary world.

The Hobbit his first book about the fantasy world of Middle Earth.
The Lord of the Rings a sequel to The Hobbit
The Silmarillion a book describing the creation of the world, the history and the battles between good and evil.

Terry Pratchett

An author who really took off in the eighties. He is the author of the famous Discworld Novels. The stories takes place on the Disc which moves through the universe being held by four elepfhants that again stands on the shell of a giant turtle.

The Colour of Magic: the first discworld novel.
The Light Fantastic: the sequel
and many more.