Tekumel (or Tékumel) is a game setting created by Professor M.A.R. Barker (Muhammed Abd-al-Rahman Barker) a retired professor of linguistics and South Asian studies at the University of Minnesota. It was originally a setting for Dungeons and Dragons, in fact it was the first published setting in 1975. (Not published by TSR, the first setting published by TSR would be Greyhawk.) The author has run his roleplaying games in it since, making it over 25 years old. Later, it spun off to become its own roleplaying game. Finally, there's been an adaption of the Tekumel setting into GURPS available on the web. So, there's at least three different well known ways to play in Tekumel.
There are an immense number of Tekumel books, so I won't even attempt to list them. There's a quite extensive list on the web, so feel free to examine the URL I will list at the bottom if you are a completist. However, to enter into the game I would recommend either starting with the GURPS edition (which is my choice) from the web if you want to just examine things a touch.
There were two novels published in the Tekumel setting by M.A.R. Barker, Man of Gold and Flamesong. There was a third novel that was being worked on, but there is no known date for him to finish the book.
The setting is set in the future by 60,000 years. There was a nuclear war in 2013 which destroyed much of the Earth's livable land (Europe, USA, and China). When civilization was rebuilt, it formed into a world-state which eventually managed to create an FTL (Faster Than Light) engines for spaceships. Eventually humans (and their alien allies) discovered the planet of Tekumel and terraformed it into a more Earth-like planet. (After they conquered the native life.) Once they were there, the planet fell into a pocket dimension.
The new planet was now isolated from the original world. In this pocket dimension, they discovered magic, gods, demons, and many other things that make up a typical Fantasy world. The culture of the inhabitants started to mutate. And because the gods demanded to be worshipped, and because they had a policy of non-interference with each other, nations were formed (the Five Empires, including the Empire of the Petal Throne of Tsolyánu.)
The background of this game is more intricate and detailed than anything I've ever seen. Every nation has the full treatment that only a professor of linguistics could ever have given it. There seems to be a heavy Asian feel in some parts, in other places a heavy Middle Eastern Feel, and yet the game has a European rennaisance focus. All in all it's hard to not feel immediately overwhelmed by the amount of detail present. (I have yet to completely devour any one background book of the game.)
In the process of double checking my facts, and gathering together information for the game, I found the following websites to be useful: