the 3rd book in the Chronicles of Counter Earth by John Norman
back cover reads..
In which Tarl Cabot, Tarnsman of Gor, sets out to discover the true nature of the priest-kings who rule over Gor with an iron and absolute discipline. No one knows what the Priest-Kings are- wheather alien or supermen. Noe have ever returned from the mountains of Sardar, traditional stronghold of the merciless rulers of Gor. Now Tarl himself is on the verge of entering Sardar.