A short write-up, Why build new worlds, which surveys the origins and of secondary creations such as Middle Earth.
One aspect of these attempts at world-building is the most detailed ones invariably borrow and reconfigure aspects of our own universe. This is obvious in The Song of Ice and Fire, and explicit in The Lord of the Rings, in which Tolkien was striving to create a mythology for the Anglo-Saxon peoples. Guy Gavriel Kay takes this tendency of drawing from our world to an extreme in works such as Sailing to Sarantium, which has numerous characters who are clearly modeled upon figures from our world’s history. Similarly, Kate Elliott’s Crown of Stars series is pretty obviously set in 10th century Germany. And she says so in the afterword of the first book if I recall correctly.
But one aspect of this borrowing from our own world is that like Tolkien there is a focus on Northern European source material. Since most of the buying public are probably white for English speaking fantasy that’s a reasonable choice. But sometimes you get an author who mines a whole different part of the world, and the result can be very fascinating. Martha Wells’ Wheels of the Infinite has issues with plotting and character development, but it’s imagining of a fantastical Angkor-like civilization is beautifully rendered.
If there is one area which I thought would be excellent source material for a secondary world, it’s the highlands of Ethiopia. I’d love to read fantasy which draws upon this land’s history, in a part because most people (including me) would not have as clear of a sense of who was based on someone real and the correspondence of events to those in our world’s history.