Sometimes library books are like buses – you wait for ages for one to appear on the shelves, and when you finally spot it the sequel is there too.

That’s what happened to me with Ruth Dugdall’s first two novels: The Woman Before Me and The Sacrificial Man.

Both feature probation officer Cate Austin. It’s an interesting choice of perspective, a less usual angle, and it is clear that the author, a former probation officer, knows of what she writes. Cate copes as a single mother, whose ex lives nearby with his new family, and she works hard at a new job, fully aware of the importance, the potential consequences, of what she decides. It all rings true, and Cate is an interesting character, but Cate isn’t the main focus.

The main focus of each book is the woman whose case Cate has to evaluate.

The Woman Before Me tells the story of Rose Wilks. She is in prison, convicted of killing her friend’s infant son in a house fire not long after the death of her own child. And now Rose is eligible for parole. But before she can be released Rose has to express genuine remorse for what she has done. But she doesn’t. She can’t. Because, she says, she is innocent.

The facts seem to be indisputable, and yet it is clear from the start that Rose’s story can’t be as simple, as open and shut, as it sounds. And it isn’t.

Cate’s role is to advise whether parole should be given. The story follows her as she meets and talks with Rose, and as she investigates and interviews all of those involved. And it follows Rose as she lives her life in prison, as she thinks about her future, and as she recalls her past.

The different perspectives work together well, and as the story emerged, with a mixture of small, quiet and larger, more shocking revelation I was engaged and I was unsettled. My interpretation of the story kept changing, and I really couldn’t decide what Rose was guilty of, whether she sinned or was sinned against, what had made her the person she became.

I should have seen the final twist coming, but I didn’t. It made perfect sense.

The Woman Before Me was dark and intriguing. The style was unpolished, one or two characters underdeveloped, but it was most definitely a first novel of huge promise.

The Sacrificial Man fulfilled some of that promise. The quality of the writing was better, the story was more complex, and the themes better developed.

This time around Cate had a different role: making a sentencing recommendation.

Alice Mariani had assisted a suicide. The suicide of a man she met in an internet chatroom. The man she knew as Smith had told her that he no longer wanted to live, that he wanted to control his own death.

The structure is the same as the first book. Cate meets and talks with Alice, and makes wide-ranging enquiries. It delves into Alice’s past. And the papers that Smith left behind added another dimension.

Alice might have been a fool, she might have been deranged, she might have been damaged. She might have been all three. She wasn’t likeable but she was intriguing, and I really couldn’t work her out.

The story twists and turns, and twice along the way major revelations change everything. They are nasty, but not gratuitous, and they do enhance the plot.

I can’t say too much about the plot, but I can say that there is a lot here to think about. Just be aware that while the story is very readable, it is also dark and difficult.

The final twist is stunning.

And it leaves me thinking that I might have found another must-read series.