I haven't finished it, but, a dense plot, predictable as to the direction it will go, but whip players up as to the path chosen.
Anna has chinks in her good-girl armor, has had them from the beginning. I don't know if this has been discussed, but could it be possible that Anna was using Jen? A relationship between Chris and a friend of hers would have caused the 2 couples to be even closer, and Anna to have a continuous source, at least of juicy details
Jen would have noticed who Chris is interested in, she wouldn't have conceived that Anna is so blind and disinterested in the interest shown by Chris, and she has turned away from them both, feeling that they are both using her.