The Green and the Gray — Timothy Zahn

Strangers Among Us
For years the Greens and the Grays have lived quietly among us in the shadows of New York, alien refugees from a war of attrition that utterly destroyed the rest of their kind. Passing as everyday citizens, each group has long believed that they are all that remain of their old world and their terrible conflict.

But now, to their mutual surprise, they have found each other, and the old hatreds and fears have once again risen to the surface.

On a cold October night, Roger and Caroline Whittier are accosted at gunpoint, and an unexpected burden is thrust upon them: Melantha Green, a twelve-year-old girl snatched from the hands of a peace coalition consisting of both Greens and Grays. The coalition had been preparing to cold-bloodedly sacrifice her in a last-ditch effort to prevent the impending battle...and it desperately wants her back.

As Roger and Caroline strive to protect Melantha, they enlist the aid of a NYPD detective and a former Ellis Island clerk. These unlikely allies have just one week to find a way to prevent New York City from becoming a battlefield for a surpassingly strange gang war, the like of which the world has never known...