The Dragon Masters

The setting is sort of similar to the Pern series (although this was written before McAffrey's Pern series.) It's a planet that is visited at long intervals by violent invasions of aliens. The people use their dragons--bred into various and sundry kinds--to fight the invaders. Unfortunately, they also spend much of their time fighting themselves.

The secret, which Vance reveals bit by bit, is that the dragons are actually descended from a few captured aliens--i.e., the Basics. The Basics have been bred into many different forms--some large, some small, but all good for various roles in combat. The aliens, meanwhile, have done similar things to their captured human slaves: they have been bred into beasts of burden and fighting animals.

I enjoyed the book. It's short and somewhat predictable, but I liked it.