Reading this series in order is not essential but is recommended. I am reading them in order and they have just been getting better and better. This is my favourite so far and this instalment is pivotal in the series. All of the preceding novels will always be the ones leading up to this one and the latter ones will be the ones that came after.
In this book Gamache’s skills were sharp, the mystery brilliantly plotted, themes about cracks letting in light superb, and the pacing was like none other so far, with plenty of unexpected twists and turns. The corruption at the Sûreté de Québec ramps up to a dangerous level and so much of the recurring characters’ development comes together so amazingly well, that it left me breathless and unable to put the book down. Gamache’s goodness and wisdom have reached a pinnacle and it makes me wonder where Penny can even take this series next. And because it takes place in the darkest month of the year, just before Christmas, Penny’s beautiful descriptions of shadow and light seemed especially poignant right now.
Chief Inspector Gamache returns to the small village of Three Pines to investigate the disappearance of a woman who was once one of the most famous people in the world but now goes unrecognised by virtually everyone. At the end of the last instalment (The Beautiful Mystery) Gamache suffered the worst betrayal yet as the corrupt management of the police force turns more people against him. The Inspector retreats to the village of Three Pines not only to investigate the missing woman, but also to seek a safe place for himself and a handful of his still-loyal colleagues, as he engages in an epic battle between good and evil and implements a strategy for seeking truth and justice.
Joanne's Reading Blog 
Totally agree with your assessment of this book – it is my favourite as well. Louise Penny’s story about the title is a beautiful addendum. Thanks!
My favorite too! All the embedded wisdom, and symbolism, tucked into a great story, with unforgettable characters.