SURVIVOR

Sphere SF paperback, 1981. Cover artist uncredited. Possibly
Tony Roberts?

"ALANNA 

was a wild savage when Jules Verrick's Christian Missionaries caught her stealing from their land. They shot her and would have killed her if not for the intervention of the leader of their cult. Jules carried her back to his house, tended her wound, and announced that she would become his adopted daughter. 

He taught Alanna to be human, at least to all outward appearances. But Alanna had plans of her own - and they didn't include becoming human. And as things turned out it was best for Jules, best for his people, and especially for Alanna. For soon they left Earth and settled on a strange, barbarous planed inhabited by warring tribes of aliens. And in a desperate bid to save the cult from extinction Alanna became something else entirely..."

This might be one of the scarcest books I own. Octavia E. Butler's "Star Trek" book* Survivor was last printed in English in 1981 (this edition) and hasn't been reprinted since. You can find it on eBay, Amazon, etc. for prices ranging from £50, to £80 (this edition), to £155 (1st edition). I didn't pay anything near that by the way, this copy was at the bottom of a box of about 70 books I bought in bulk for £25. Everyone I know said sell it! But who's going to cough up that much for a single paperback?

*It's got nothing to do with Star Trek. Butler simply disliked the novel and referred to it this way because it featured humans and aliens mating.