Witherwing, by David Jarret; Warner Books, September 1979. This is one of those books that is more interesting for the glimpses of the underpinnings of the setting than for anything that the characters do or that happens to them. Enjoyable in an abstract way, for someone interested in Swords & Sorcery fiction.
Sword & Sorcery Roleplaying
I read Witherwing because, like Gardener F. Fox's Kothar, it was in the reading list of Sword & Sorcery books in the Sorcerer & Sword supplement for the Sorcerer roleplaying game, both which were written by Ron Edwards. It was listed as one of the references for the “Mutant-Future” example setting. Sorcerer & Sword (as is typical of Sorcerer and its supplments) approaches Sword & Sorcery very differently from most roleplaying games: it concentrates on the original Sword & Sorcery authors (for instance, it considers only those Conan stories actually written by Robert E. Howard, which by now is probably the smaller part of the Conan series), includes RPG mechanics only for things that can't be handled by concepts from other RPGs, ignores the loot and level basis of the widespread monster- killing style of roleplaying,  and only including things that will be directly useful in play. This later means, for instance, that there is no detailed, specific background material included, since it takes than stance that most of that material is never used in play. Instead, there are examples of starting with a minimal setting and creating the details during play.
In any case, I thoroughly recommend Sorcerer & Sword for anyone who is playing grim, intense, Sword & Sorcery-based roleplaying games.
And as source material for “Mutant-Future” games, Witherwing is full of ideas.
I've read more of Guardians of Order's new Tékumel: Empire of the Petal Throne roleplaying game and it is very good. I've noticed a few typos so far, but nothing serious, and the game itself seems pretty clearly explained. Moreover, the world background material is very good. The old Swords and Glory, Volume 1, the Tékumel Sourcebook,  still has more detailed information, but the new game provides a distilled essence that provides enough detail for newcomers to understand Tékumel and have some idea of how to play a character from the setting. It concentrates on Tsolyánu, which is the area of Tékumel on which we have the most information, and has information for the more common non-human races as well.
Mechanically, the system is derived from the Guardians of Order Tri-Stat game system, although it uses 1d10 instead of 2d6 or 2dX, and it uses 6 stats, instead of 3. The list of attributes available to starting characters has been simplified and specialized for Tékumel and the skills list has been expanded and specialized for Tékumel. Magic is handled separately with a system that seems to capture the flavor of Tékumel's ritual and spell-based magic very well. Character creation includes those social aspects that dominate Tsolyánu culture: clans and careers in the legions, temples, or government, as well as an extensive subsytem for dealing with resources, a subject which can be complicated in Tsolyánu since property tends to be owned by the clan rather than individuals, as well as smaller subsystems for dealing with teamwork, respect, and favors.
Overall, I'd say the game is somewhat crunchier than BESM (there are stat requirements for weapons and some careers, and spells can have multiple levels with different enhancements and various specialties), but much, mucher lighter than Swords & Glory, Garásiyal, or 3rd edition Dungeons and Dragon.
Verdict so far: the new game makes me want to play Tékumel: Empire of the Petal Throne, and provides the right material so that it's possible to see how to do so.