As of now, you can download Horn Crown by Andre Norton for free from Amazon (US, ES). This is the book synopsis:
Book seven of the Witch World - High Hallack series. When the Elder People deserted the Witch World, the cosmic Gate opened and human households migrated in. Though they found the land abandoned, the roads empty and the shrines unattended, they soon became aware that the Old Gods still retained their dread powers.
Elron the Clanless and Gathea the Wise Woman were drawn unwillingly side by side into unexplored realms. Each had a separate quest but their fates were linked by the unknown. Each sought an unholy alliance; each was in conflict with the other.
Andre Norton has at last written the seminal Witch World novel - the book that tells of the first coming of mankind and which sets the basis for the entire series. HORN CROWN is truly the keystone arch of the Witch World legend.