There's some really unfortunate racial attitudes in here, courtesy of the time it was written, but what sets this treasure-hunting story apart is its focus on cryptography. It's all laid out very carefully, as the structure of the cipher is slowly untangled for benefit of the reader. (A little too slowly, perhaps, yet cryptography is an interest of mine so perhaps I have the advantage over some other readers here.) The subsequent discovery of solution leads to a series of clues which leads to treasure, as it so often does, and it's the kind of thing that makes the thought of treasure-hunting appealing... if you can get over the hours and hours of digging and accusations of madness, that is.