The Word For World Is Forest: Ursula K. LeGuin

The Asthsheans are, literally, little green men, dwelling peacefully among the forests of their world. The culture places great emphasis on the importance of dreaming: their experiences while dreaming and awake are both considered to be real and valid. Their society is peaceful and gentle: murder and war are unknown. To most of the human colonists, the planet - which they know as 'New Tahiti' - is there to be plundered of its valuable timber. The Asthsheans are contemptuously known as 'creechies', to be exploited as slave labour, to be pushed out of their homes and slaughtered without thought when they protest. A few men, such as Raj Lyubov, try to stand up for the Asthsheans - but they are greatly outnumbered by the crazed Colonel Davidson, to whom the only good creechie is a dead creechie. Inevitably, despite the efforts of Raj Lyubov and those like him, the Asthsheanshave to learn to fight back against their oppressors. And they do - causing changes in their traditional way of life forever.