If you think this is gruesome, or that it's only other living creatures that get treated this way, I'll point you to King Leopold 2 of Belgium and his treatment of the congo.

https://www.amazon.com/King-Leopolds-Ghost-Heroism-Colonial/...

Warning: this book will probably give you mental trauma.

Friends of the Congo: http://www.friendsofthecongo.org/films-a-videos.html has some great resources about this.

Another classic is Heart of Darkness (although a fiction, it tells much about colonialism)

King Leopold's Ghost is also a great read: http://www.amazon.com/King-Leopolds-Ghost-Heroism-Colonial/d...