This little mystery is nicely turned. The author proportions the story very well to the book's length. It is not a particularly long novel. The plot is pretty simple and straightforward -- there are no subplots that I can think of.
The author is very effective in capturing the character and mannerisms of Holmes and Watson. He is also highly knowledgeable about the London of the late Eighteenth Century that Holmes inhabited. His pages of "notes" at the end of the book show that he is quite a Holmes scholar, which may explain how he succeeds in recreating the flavor of the original fiction so well.
Action scenes are very well rendered. There are several of these, including a major gunfight. The gang of nine that provides the villains is very similar to secret societies in several of the original Doyle novels.
One possible shortcoming of the book is a void in characterization. For example, the arch-villain of the book is a child-molesting Catholic Cardinal. This same Cardinal is also deeply involved in the theft of the Vatican Cameos, which poses Holmes his challenge in the book. However, this clerical figure is little more than a cardboard cutout. He never emerges as a character at all.
To say more about the book's characters, an American detective from New York, who assists Holmes, comes across somewhat more convincingly, as does good old Lestrade. Inspector Lestrade in this story is less the fool and more an underappreciated and mentally sharp guy. He is really a true friend of Holmes, not just the jealous dabbler as he so often appears. Unfortunately, many imitators of Doyle love to make Lestrade a pathetic fool or buffoon. I like this "sharp" version of Lestrade much better.
This novel is not War and Peace. But it is a very nice little piece of work, wonderful for reading in the tub, on a plane, or while nodding off to sleep. Seriously, I do plan to try and get my hands on more of the several Sherlock Holmes novels that this author has penned.
If you purchase the book, you probably will not be disappointed.