Poul Anderson 
Star Ways 
Psychotechnic League Book 1

"Five of our worlds are missing". That was the essence of the report that shocked the galactic Nomads at their annual meeting. For each of the mighty star-ships reported vanished was a world of its own – a man-made, self-sustaining city-state housing thousands of people. The Nomads themselves were an unplanned by-product of man’s conquest of the stars. They were the gypsies of the distant future, the restless rovers of outer space. But to Joachim of the peregrine tey represented a way of life that was to be dearly defended. So it fell to him to make his own world-ship the bait in a cosmic trap set to catch the galaxy’s unknown foemen!