I enjoyed Carr's The Alienist and The Angel of Darkness very much. Alienist is an archaic word for psychologist, and the detective Carr created is a psychological expert in the depths of the human mind. Those two previous works were works of psychological detection; this work is 'commissioned by the estate of Sir Arthur Conan Doyle'. I can't imagine they expected what they got; a straight-up pastiche that's as unpsychological as they get.
We live in an age where readers expect some kind of character arc or elucidation in a book; Carr wipes away the tiresome conventions of character development and gives us a no-nonsense mystery with a bit of superstition tossed in as a kind of tip of the hat to Doyle's previously mentioned weirdnesses.
The plot involves a murder coinciding in location and method with the famous murder of Queen Mary's 'Italian Secretary'. This requires tedious amounts of exposition and soliliquoizing from Holmes. There is an interesting looking sinister character introduced early on, but Carr for some reason decides that suspense would impede him plot and assigns all their roles of villain, victim, saint, without much ado or confusion.
In the end he even fails to come up with a plausible explanation for a key part of the murders.
It's beyond imagining how a book with this many explosions, train wrecks, gunfights, brawls, and ghosts could be so boring.