Boston communists say they want a revolution. Now they just have to agree on how to bring it about. A pimple-faced teen with Coke-bottle glasses and a self-satisfied smirk stands near the doorway of Harvard Square’s tiny Revolution Books. He’s skinny and draped in a large black T-shirt adorned with a full-color portrait of Joseph Stalin, the iron-fisted despot responsible, directly or indirectly, for the deaths of millions. "Stalin is my friend," the kid says, at once cockily and geekily, as he stares at the floor. "Yeah, yeah, but we can do better than Stalin," says the guy behind the...