The Mote in God's Eye

By Larry Niven and Jerry Pournelle

Set in the far future, at a time when people have spread out into the galaxy, by virture of the invention of an almost instantaneous starship drive, the Alderson Drive. Another piece of priceless technology is the Langston Field, a kind of energy field used to protect ships and cities from attack.
Humankind had never found intelligent aliens before. So news of an alien ship required careful investigation. Sent out to meet the ship is the MacArthur, a battlecruiser. But the alien ship fires on the MacArthur and the alien ship is then captured, but no one is left alive on board.
So the MacArthur and another ship, the Lenin, are sent to the system where the alien ship originated. The MacArthur is to make contact and the Lenin is to insure that valuable human technology does not fall into alien hands, even to the point of destroying both the MacArthur and itself.
Upon reaching the asteroid belt around the home planet of the Moties, as they are being called, the MacArthur is approached by a Motie mining vessel, manned by a lone Motie Engineer and a bunch of small Moties, called Watchmakers. The Motie engineer consents to board the MacArthur and brings along two of the small Moties. These Watchmaker Moties are tinkerers and have a knack for improving the workings of machinery and tools. The humans mistakenly dismiss the small Moties as mere pets of the engineer and are not too concerned when they get loose on the MacArthur.
Meanwhile the ships move in closer to the Motie home planet and are greeted by a ship carrying a delegation of Moties assigned to communicate with and study the aliens (humans) entering their home territory.
At first, it seems like the Moties are gentle and welcoming and many of the human experts feel they are mostly harmless. But some have reservations, especially when a plague of Watchmaker Moties irrupts aboard the MacArthur.
What the humans don't know and the Moties don't want them to know, is that the Moties have a secret, a secret that could threaten the very survival of the human species.

This was a pretty good story, if rather long. It has a kind of sad ending, as the Moties secret results in the humans blockading the Motie system to keep the Moties penned up. Moties are very appealing aliens and it is too bad the authors couldn't think of a happier ending for them.