Paul Anderson.
On the back it has a little text that sums up the start of the novel:
Stolen Birthright
His father was Imric the Elf Earl. His mother was a captive princess of the Trolls. But Valgard the Changeling was raised as the son of Orm in the Lands of Men. His mortal foster parents never knew that their real son, Skafloc, had been stolen at birth to be reared at manhood in the twilight fields of Faerie... they never knew that the curse that had brought them Valgard had even more heinous magic to wreak. But when Valgard learned the truth of his heritage, his vengeance knew no bounds. And so it was that he set forth to destroy his double - unaware that the gods had other plans for an Elfin mortal and a changeling born to kill!
SWORD-AND-SORCERY WITH A VENGEANCE!
It's a good, short read.