In the world of film, it's called a docudrama, I think - or, if satirical, a mockumentary. In the much older world of written fiction, the narrative device of presenting a story wholly or partly in the form of documentary 'evidence' is well-established. It's a device I enjoy very much, and thus I liked Minette Walters' use of it, in the form of various letters and statements to authorities. This is a book where we have multiple characters giving unreliable narratives. Walters doesn't indulge in an ongoing detective figure whose insights can be relied upon. There are several mysteries in the past - two deaths, possibly/probably linked - and one injustice that cannot be remedied, only exposed. It's all buried under mounds and mounds of English reticence about the nasty underbelly of domestic life - physical and sexual abuse, twisting people's personalities throughout the rest of their lives.
I found the evolution of the central investigating characters, in their growing understanding of their own particular strengths and weaknesses, interesting - but the solution of the mystery itself relied (for my taste) too much on constantly changing time/date/place evidence. Frankly, I got tired of trying to follow, which is not usually the case with me. I won't spoil, but it all made sense in the end.