Believe it or not, this classic is an action/suspense piece with the what might be the first novel with spies, greed, gun-play, high-stakes poker and a beautiful unflinching girl who knows how to load a firearm during a high-speed chase.
It is set in early 1800's and concerns a young man who meets his estranged, much despised father--a man who describes himself with the title of the book--and learns there is more to the man than he's been led to believe. The plot revolves around securing a certain document penned with the signatures of traitors to Napoleonic France--the government of which is pulling out all stops to get their hands on it.
The occasional disparaging use of the "n" word in reference to a good-man-Friday, who is one of the men's slave, is almost shocking as were they not there I might have forgotten I was reading a book written nearly 100 years ago.
I recommend The Unspeakable Gentleman highly. It is a fast and fun read, comes in at about 250 pages and is available for free through Project Gutenberg at gutenberg.org.