The weakest book in the series. There isn't much plot and no mystery at all The human characters are barely wallpaper, and the cat characters aren't much more than that. What there is too much of is the Netherworld fantasy element. I don't want to read about cats having existential crises and going all pitiful and moping about the lost realm.
Reviewers who complain about talking cats are reading the wrong series. These are magical cats that can talk and help solve mysteries. That's the whole basis of the series. If you think that's dumb, then go away and read something else.
When the author gets the fantasy / mystery balance right, this series is interesting and fun. If this were the first book in the series, I would be outta here. Given the quality of the preceding stories, however, I'll hang in for one more as soon as the lower-price paperback Kindle version appears.