When you think of Jack London, you probably think of works like Call of the Wild -- yes, there's something fantastic about the work, but in the end it's not something you'd want to shelve with the SF or the Fantasy.
It's conservative, and thoughtful, and has a very real understanding of the world. This is something different. Oh, it's got London's understanding, but it was written before he changed his politics -- The Iron Heel gives us a grim, SFnal, and socialist vision of the world endlessly at war -- Vietnam-style war, at that. Fascinating, thoughtful, and exciting stuff from a real master.
"Still more astonishing is the genuinely prophetic vision of the methods by which the Iron Heel will sustain its domination over crushed mankind." -- Leon Trotsky