Red Rubber: The Story of the Rubber Slave Trade on the Congo by Edmund Dene Morel (

Book scan at https://archive.org/details/redrubberstoryof00more/page/n7/mode/2up?ref=ol&view=theater

and also at Google books https://www.google.com/books/edition/Red_Rubber/pg4QAQAAMAAJ?hl=en

not on Gutenberg

librivox https://librivox.org/red-rubber-the-story-of-the-rubber-slave-trade-on-the-congo-by-edmund-dene-morel/