Editions

Empire of the Petal Throne
The original 1975 Tékumel rules set

Swords & Glory
Strong background material, originally meant as a three volume set, but the 3rd volume - the Referee's Guide - failed to be printed. (1983-1984)

Gardásiyal: Deeds of Glory
(1994)

Mitlanyál
Two volume sourcebooks that was originally one [called Mitlanyál], but was split (2004).

Volume 1: Tlomitlanyál; Gods of Stability

Volume 2: Tlokiriqaluyál; Gods of Change

Tékumel: Empire of the Petal Throne
A full system and information for creating characters and running the game in one hardcover, followed by a plague of licensing issues. (2005).

Béthorm: The Plane of Tékumel
The most recent incarnation of "Official EPT". (2014). The system is a bit complex and weighty with everything being a skill that must be learned, rolled against, and even known by the players to roll against with more difficulty if the character doesn't poses some skills.

Unofficial Rules
Additional alternate rules can be found on Brett Slocum's site and The Eye of Joyful Sitting Amongst Friends site

AD&D Variant (Wizards of the Coast)

Humanspace Empire
Adventuring in the Humanspace Empire, before the Time of Darkness. A time of high technology - think Star Trek & Star Wars.