I was disappointed by this book. I made the mistake initially of thinking it was the first in the series, and found myself a little lost, but that was not the problem, ultimately.
I stopped reading halfway through because the story became incredibly dull and plodding, as if the author only had a much shorter story to tell, but was forced by his publishing company to stretch it out to several hundred pages. The action was simply not happening, and the "mystery" started to become slightly transparent, despite a promising opening portion of the tale (the beginning is truly frightening and makes a person eager to keep going).
Also, the main characters do not have a convincing relationship to each other. Dr. Bell and Doyle are slightly two-dimensional, and Bell, especially, comes across as a sort of mechanical imitation of Sherlock Holmes, rather than a fully fleshed-out character. It makes one wonder why Doyle would be spending time with him. This book certainly does not capture the fascinating friendship that the real Doyle wrote so well between his two famous protagonists.
Another thing that annoyed me was the author's much-too-frequent use of a little writing trick where he describes something that can't quite be seen looming towards the narrator and then reveals it to be harmless. It was initially effective, but lost impact after he had used it several times.
On the bright side (or the irritating side, depending on your viewpoint) the author weaves in many references to canon that are entertaining for a Holmes fan to pick out.
I don't leave books unfinished often, but I had no more time for this one, unfortunately. I loved the concept, but the execution was disappointing, despite a promising start.