A long short story or a short novella, this offering to fans of the Russell / Holmes series fits takes place in the timeframe of the first novel in the series, The Beekeepers Apprentice. It is essentially in two parts. The first part deals with the meeting of Russell and Holmes from the point of view of Holmes. The second part recounts an episode which occurs later in the timeline of the novel, which Russell could not have known about because the novel is written in the first person, from Russell's perspective.
The narrative shifts from first person to third person. In principle, this is not something to which I object. However, the technique is not very effective here, possibly because the work is so brief that the switch from first person to third person makes the narrative feel choppy.
Still, this is a delight for fans of the series. (Although even fans may find the "Twitter interview" at the back of the work something of a yawn!) Those who are waiting to be converted should read The Beekeepers Apprentice first.