Margaret Weis: Elven Star [1991] paperback

Pryan, the Realm of Fire – a world where never-ending sunlight and plentiful rain have given birth to a jungle so vast that humans and elves live in the trees and only dwarves live anywhere near the ground. From the treetops the aristocratic elves serve as weapon merchants to the other races, whose incessant warfare sends a steady stream of profits and essential resources skyward. Passing through the Death Gate, Haplo is sent to sow chaos among Pryan’s inhabitants. But the constant wars have already achieved this. The generations of dissent and race hatred will not heal even under the threat of annihilation at the hands of the legendary tytans. Crying doom and armed with a magic as powerful as Haplo’s own, a human priest and his dragon ride in the vanguard of destruction. Now Haplo’s salvation lies in his ability to overcome the tytans – but not even he knows the way to conquer these renegade giants.