Greendaughter by Anne Logston
"Long before Shadow was born, the Heartwood was a land of warring clans of elves. When human lord Sharl comes to build a great city near the forest and abducts Chyrie and her mate, Valann, to gain safe conduct through the forest, the elves learn that an even greater enemy is approaching -- an army of barbarians sweeping down from the north. Now the elves must make peace, not only with each other but with the hated humans, to unite against a threat that may destroy the city and the forest together."
Mass market, a nice copy!