It’s hard to say much about any murder mystery without giving away key aspects of the plot. It’s also hard to evaluate any novel as a whole without taking the ending of the book into account.
That being said, I did enjoy reading Original Sin until the ending. It’s good enough reading that I wouldn’t want to spoil someone’s potential enjoyment of reading it by discussing the ending, but I have to say that I did find the ending somewhat less than satisfying, which colors my impression of the book as a whole.
I’m not sure my disappointment in the ending had to do with the way the resolution of the plot was constructed and how much with the events themselves. On the one hand, I found actions by certain characters somewhat lacking in believability. (Yes, it could happen that way, but how likely is it?)
I don’t know if it’s just me, but it seems that believability is more important to a mystery than some other genres. The answer to the puzzle has to feel like it fits, not that it is forced in like a square peg in a round hole.
On the other hand, part of my unhappiness with the ending has to do with how sad it is for so many people, and that is a very believable aspect. Life is full of pain, much of it caused – as it is in this novel – by other people, and all too often by those who should be a help rather than a cause of hurt.
In that, the title is simply expressive of life as we know it. Whether one accepts the idea of original sin in traditional Christian theology (though there are various interpretations of it even within mainstream Christianity) or not, experience tells us that people tend to act selfishly and often cruelly toward one another even in the most “civilized” societies.
What is missing in P. D. James’ depiction of the results of sin, at least in this novel, is any sense of redemption. The previous book I read by James (previous in the order I read them, not that she wrote them), A Certain Justice, acknowledged the limitations of human justice, but seemed to offer more of a sense of hope in that we do what we can toward that end.
In Original Sin I see little sense of hope. It is not that life is inevitably painful and depressing – there is a sense of “things could have been better, if …” But people made the choices they did, and the end result is mostly pain and sadness.