The Little House by Philippa Gregory
It was easy for Elizabeth. She married the man she loved, bore him two children and made a home for him which was the envy of their friends. It was harder for Ruth. She married Elizabeth's son and then found that, somehow, she could never quite measure up... Isolation, deceit and betrayal fill the gaps between the two women and between their two worlds. In this complex psychological thriller, Philippa Gregory deploys all her insight into what women want and what women fear, as Ruth confronts the shifting borders of her own sanity.
I read the book all the way through, but I have to admit that I found it horrifying (that was the intent, I think) particularly in as much as what happens to Ruth could really happen. shudder.