Jack Vance 
Asutra 

Gastel Etzwane had held total power in the Land of Shant: the brooding musician, turned man of action with his corps of Brave Free Men, rid the land of slavery and alien invaders in one masterful stroke-only to discover the greatest threat of all — alien parasites of inconceivable intelligence and unfathomable purpose. Their expulsion from Shat has been celebrated by all, but Etzwane is uneasy. For wild rumours come out of the hidden land of Caraz-and once again the musician must don the cloak of action, setting out on a dangerous quest for knowledge that will take him farther than he ever wanted to go: to the grim reaches of a hideously alien world, there to learn the secret of the Asutra!