So I didn't let it bother me that there was an "e" in the word "judgement" in this title, but as I'm writing this, it IS bothering me. It's a book written by someone in the UK, so I'm going to stop thinking about what the correct spelling is right now.
This book was weird. In the first two pages, Rendell tells us exactly who is going to get killed, who is going to do it, and how it's going to happen. So this isn't a mystery and it's not exactly a crime procedural, either. So I was sort of puzzled about what this book was going to actually be about. I found it on a list of great mystery novels and it clearly wasn't a traditional mystery, so...
It's basically a commentary about the class system. The killers are lower-class and you see shame, anger, and defeatism from those characters. The family that is killed is a higher class (wealthier and educated) and you see the carelessness with which they are unaware of their privilege and their often unintentional mistreatment of their servants. And I liked the pointedness of this conversation on class.
I didn't actually care for the writing, though. The author seemed to use language to obscure meaning. I had to read the first two pages (when we learn about the crime, the victims, and the circumstances surrounding the crime) about five times before I could figure out what was going on. Some of that was because I just couldn't comprehend that the author told us all the main plot points at the very beginning, but some of it was because the writing was unnecessarily dense and circuitous. I got into the swing of things and figured it out eventually, but I didn't really warm up to the writing style. I did like the author's witty one liners, though. One of my favorites, used to describe one of the murderers, was: One wonders what Joan Smith would have done with children if she had had them. Eaten them, perhaps. Ha ha ha.
So I will recommend that you read this book if you enjoy the psychology of killers or snarky British narrators, but if you just want to sit down and curl up with a good whodunnit, this probably isn't for you.