When Demons Walk is considered a stand-alone novel however it utilizes an established magic system that may be overwhelming to new readers unfamiliar to the series.
For a more rounded and enjoyable experience I would recommend this book be read last in the series as opposed to third. The story itself is a fun mystery with interesting new characters and a new region of Sianim to explore.
The Sianim series is a set of Patricia Briggs' earliest work. It lacks the caliber of her popular Mercy Thompson series however I believe this book is one of the more polished out of Sianim. It that shows a greater grasp of prose, description, dialogue, and focused point of view. The subtle romance is also threaded through the story better. When Demons Walk is one of the better composed books of Sianim and I found it a good addition to the series.