I read cozy and historical mysteries, a bit of Paranormal/UF, and to mix it up, I read science and gardening books on occasion.
At the start of book 10 A Little Night Murder (I think) it's obvious there's some backstory missing; something that was going on between book 9 and 10. This short story fills in the gaps. It's not strictly necessary, but for a reader invested in the series, it's satisfying to have.
As a bonus (because not all between novel novellas are equal) this is a very well written, fully formed story about Mick's struggles to get out of the family business and stay legit. Martin always wrote Mick as a real person, struggling against his upbringing to be better and this story gives the struggle centre stage.
Really well done and reminds me why and how much I miss this series.