The Tekumel Sourcebook is now available from DriveThruRPG! The Sourcebook contains a vast array of information about the world of Tekumel. This is a reprint of the original Gamescience edition, with the addition of an index and a glossary.
Here is the Foundation blog post: http://wp.me/p2elQO-aj
Here is the DriveThruRPG link: http://www.drivethrurpg.com/product/1418