Welcome, grendelwolf!
The single best source for setting information (no game mechanics) is the Tekumel Sourcebook, which was volume 1 of the ill-fated game Tekumel game called Swords & Glory. It is by far the largest compendium of information. Lots of stuff about the culture, the societies, the material goods, the legal and social systems. It was first published by Gamescience in the early 80's, and you can still find those editions around. This one volume was later republished as three separate booklets by Different Worlds. The text is the same, though some of the art is different and the layout is much easier on the eyes.
Tita's House of Games has both re-prints of the one volume version and copies of the three booklets:
http://www.tekumel.com/tita/swords.html
This book doesn't have monsters, spells, magic items, or game mechanics of any kind.
Another place to start is Tekumel: Empire of the Petal Throne, the most recent Tekumel game. It was authored by freelance fans, and published by Guardians of Order (now out of business). It was intended to be a free-standing single volume game. In order to do that, they had to leave out coverage of the regions outside the Empire itself, include only a portion of the creatures, leave out some of the powerful temple spells, and only mention types of magic items (though there's a good system for generating your own. It has good coverage of the Empire of the Petal Throne itself, lots of valuable interpretive text, and there are in-game mechanics for important things like Teamwork and Respect. The game system is ok, but has some serious flaws, especially with the magic system.
I think Tita's has some of these too.
The closest thing to a really complete Tekumel game is Gardasiyal. It was a boxed set with three volumes (combat , spells, and gm info). But even it required additional material. For chargen you need Adventures on Tekumel Vol 1 (first in a series of solo adventure books) and for the monsters you need the Tekumel Bestiary. Also it doesn't have all the setting info that the Sourcebook has.
Regarding maps, the Gamescience 11x17 maps are the way to go. The Gardasiyal map isn't as detailed, covers a smaller area, and introduced some errors in city sizes.
I hope this helps. I'll try to keep an on this thread, you are welcome to reply with more questions.