By the Light of the Green Star — Lin Carter

Miscast is the role of assassin, inhabiting the stolen body of a stalwart savage, the star-wanderer from Earth found himself in dangers beyond even his wildest imaginings.

His friends had fled, his princess with them, and awaiting him were the islands of the sky with their merciless masters, the prowlers of the dark tree bottoms with their horrible steeds, and the treachery of his comrades in captivity.

But this was what he had left his safe home on Earth for--and his adventures BY THE LIGHT OF THE GREEN STAR, however knife-edge, were the very staff of life to him!