Matters of Form — Scott Wheeler

The Shan, immortal alien shape changers, who could offer mankind both the freedom of the starways and an eternal lifetime to explore the universe. The Phoenix Group, shipwrecked Shan, separated from the last remnants of their race, working secretly on Earth to bring human technology to the level that would enable them to repair their own spaceship and rejoin their people on the last Shan citadel world of Haven. But time was running out for Shan and human alike, for there was another race of space-going aliens loose in the galaxy, a race of destroyers on a deadly hunt for the Shan. And even if humans and Shan allied against this foe, it might already be too late for either race to escape the wrath of these conquerors from the stars!