A Dark Dividing is a 2004 horror novel from the pen of Sarah Rayne, who both I and Wikipedia have never heard of (which is terribly perplexing, considering the following: that the novel was released in 2004, it wasn't her first and she's also British like myself). It's being re-released for the U.S. market and that is why it's come to my attention.
The plot involves a complicated family history that a hard-bitten journalist tries to get into and two sets of conjoined twins from 80 years apart (the first set were born at the turn of the 20th century and the second set were born in the 1980s or thereabouts) that are connected somehow. The present scenes are written in the third person and follow the story in general, whereas the "diary extracts" from the 1900s follow one specific woman's story in the first person and can get a bit tedious at times. I much preferred reading the present ones and honestly considered skipping the past segments.
It also has running themes and family connections throughout, which hint at a greater book than you get. For instance, both periods have husbands that the wives are not happy with who use their children for their own ends. There is also the strand of family tying the whole book together, although this only really becomes apparent around about page 300 or so.
A Dark Dividing reminds me of the kind of horror thrillers that I used to get from the library when I was about 11 or so, (Point Break/Point Blank or whatever the hell the range was called) as it uses a similar type of story structure and horror except it has more grown-up protagonists. It also sets out (whether intentionally or not) to teach the reader about conditions in Victorian times and about conjoined twins, all while building to a powerful ending where all is revealed.
Well, that's the idea, but where it falls down is the fact that the ending itself doesn't really work. What was no doubt intended to be a murder-mystery style summation of the events of the book explained to one of the characters who wasn't there just comes across as a recap of the book you just read to the reader who was there. As a result of this, the last 15-20 pages of the book could be summed up in story terms with a sentence, thereby making them unnecessary: "I explained the events to [character] for a long time." That would've sufficed. To add insult to injury, the reader doesn't even get closure about one of the relationships that arguably forms the backbone of the book (the 1900 diary extract lady and her lover).