Unlike the first three books of the series, Holmes and Watson are secondary characters here with Shadwell Rafferty taking center stage, which means this book will be a disappointment to all but those readers who've developed as much of an attachment to Rafferty as to Holmes.
In fact, you'll need to go through more than 1/3 of the book before the Baker Street duo actually arrives in Minnesota, and even then, you'll never be reading the Watsonian-style chronicle you're accustomed to. Instead, Watson's contribution is presented in the form of hastily written journal entries, which are full of abbreviations parenthetically explained by Millett in his role as "editor." The other parts of the story are filled in with standard prose---more or less from Rafferty's point of view---but not written in his voice.
This creates a somewhat disjointed narrative, and worse, an irritating rhythm, the brief, fast-paced journal material always being followed by the longer, plodding prose.
It seems rather obvious that Millett used this book to shift his focus from Holmes/Watson to Rafferty and his partner, G. W. Thomas (Thomas is shown in a way that unmistakably parallels Watson), and I wouldn't be the least surprised if Millett evolves the series into one of Rafferty's own.
I'd be sorry to see a decently written Holmes series end, but I do like Rafferty, and think Millett deserves some praise for his creation and development of the character. I've also grown to admire Millett's ability to weave fictional characters and elements of mystery into his state's history (If you've been reading this series, but skipping his historical notes, you've actually missed a bit).
Ultimately, I rate the book only three stars because it doesn't have enough Sherlock to appeal strongly to a Sherlockian, and because if it is indeed a transitional volume between a Holmes series and a Rafferty series, it feels like an awkward transition.