Henry James the author and Sherlock Holmes the character. It's an unusual pairing. What puts the two together? How did Simmons even come up with that idea in the first place? The story is ultimately a Holmesian story, complete with disguises and recurring characters, but what made the author think to pair him up with an American writer living in England, and then transport them both back to the US?
At first, I thought the story might be similar to something Tim Powers would write, with the factual events of the story being actual facts, and the rest of the story explaining what went on behind those facts. I didn't do a great deal of research, but it appears that James didn't even travel to the US during the time in which this novel is set. It's just a Sherlock Holmes story featuring an American writer. I don't know how else to explain it.
James' inclusion in the story, though, isn't just a random assignation, though; the mystery of the novel starts and ends with the writer's acquaintances, and that James gets involved is a part of the story and the mystery. In short, though, Holmes is investigating a suicide that he -- and at least one other person -- believes to be a murder, and during the course of the investigation, he's also trying to determine whether he is a fictional character or a real person. The second point is more theme than plot, but it comes up often enough so that the reader doesn't forget that's part of the story, too.
I don't know Henry James' work well enough to speak to how well Simmons captured his voice, but Samuel Clemens and Sherlock Holmes are captured extremely well. The point of view shifts between James and Holmes throughout the book, and the voices are distinct in each part. Clemens is an incidental character here, but when Simmons gives him a voice, it's in the one you would expect to hear. The same is true of Teddy Roosevelt, who also appears as an incidental character.
The thing is, James, Clemens, and Roosevelt were all real people, and Holmes is a fictional character. It seems like just an author's lark to throw them together to see how events would unfold around them, but there are moments where the characters become self-aware to some degree, and then the story starts to feel meta. That feeling isn't the point of the story, but it intrudes just enough to feel like the author is winking at us from just off-screen, and it was disappointing. It wasn't quite Stephen-King-writes-himself-into-his-own-story disappointing, but it felt like Simmons was being dishonest with his readers.
I liked the story, and the detail that Simmons brought to the story. It's no secret that Simmons does a lot of research for his novels, and he brings that research into the narrative, but he manages to do so without the story reading like it's a lesson. It's wrapped into the story well enough to be informative without being overbearing, which is impressive when you stop and realize just how much detail he's putting into the story. The mystery feels like it could exist side-by-side with Doyle's stories, though to be honest, I haven't read enough of the original Holmes stories to be able to compare them that well. Based on what little I do know, though, it feels like a Holmesian story.
Something else to note is that Samuel Clemens is featured in this book, as well as in Fires of Eden, which also featured Cordie Cook and Mike O'Rourke from Summer of Night. It makes me wonder if Simmons, who does a little Stephen King-like world-building of his own among all his novels, has decided to make Mark Twain a part of the world that includes the Borgia Bell. If so, it's an intriguing thought, especially with his sly references to people as characters, and characters as people, that he brings up in The Fifth Heart.