Actually, most of the slang in this book surrounds the criminal world of counterfeiting versus cowboy-speak. Criminals can spot a cop easy from the way he talks; hence, the cops would hire ropers, undercover informants who worked for the police. Excons usually made the best ropers because they had the crook lingo down and could exchange crime stories. Lewis Swegles was the roper hired to help the Secret Service in the grave robbing plan by becoming a part of the thieves plot. If he hadn't become involved, the outcome of the theft might have ended up differently.
The story's twist comes from the plot to negotiate not only ransom money for Lincoln's dead bones, but demand the release of an important counterfeiter from jail. During the Civil War more than half of the circulating paper currency was counterfeit. That's right, half! The government decided to put an end to it and actively went after Ben Boyd, the genius behind making fake money plates that made many crooks rich. Once the government lassoed Ben, the counterfeit business fell apart as he did jail time. The grave robbing scheme was hatched by a big counterfeit crook who had lost so much business he thought he could exchange Lincoln's bones for Ben.
Sheinkin does a good job describing places allowing me to visualize locations and the place of the grave robbing crime. The Memorial Hall's poor ventilation and dripping ceiling is suspenseful, "The air inside was thick with cold moisture and the stink of rotting wood. Water seeped through the stones above, dripping on the men, pooling the muddy floor." The historical quotes add tension, '"The sun had not been visible during the whole of the day," he [Powers] remembered, "and thick clouds hung like a pall over the earth, making it so dark...that a man could scarcely have seen his hand before him."' I liked the irony of the robbers looking at the marble sarcophagus that said, "Lincoln... WITH MALICE TOWARD NONE, WITH CHARITY FOR ALL." The black and white photos spread throughout the text enrich it and help with visualizing the crooks and setting.