Zane Grey 
The Rubber Hunter [EPUB ebook] 

No one in Iquitos knew him by any other name than Manuel. He headed the list of outlaw rubber hunters, and was suspected of being a slave hunter as well. Beyond the Andes was a government which, if it knew aught of the slave traffic, had no power on that remote frontier. Valdez and the other boat owners, however, had leagued themselves together and taken the law into their own hands, for the outlaws destroyed the rubber trees instead of tapping them, which was the legitimate work, and thus threatened to ruin the rubber industry. Moreover, the slave dealers alienated the Indians, and so made them hostile.