The Martian Chronicles by Ray Bradbury
This series of beautifully written short stories is an appeal to humanity's better nature. First published in the 1940's it was the first book to take human colonisation seriously.
Set at the end of the last millennium, humans go to Mars and discover life. Exploitation versus conservation is a key theme in the book and the focuses are usually the small scale individual battles of concience that the characters go through in their interactions with the red planet.
Bradbury also manages to make the Martian Chronicles a cracking adventurous read at the same time.