Mercury is the name of a very special horse—so special that Viv has sacrificed all of her ideals for this horse, which she does not even own.  Like Gone Girl, this novel contains Donald’s perspective, then Viv’s, and then goes back to Donald’s.  These two are married with children, and their marriage starts to go off the rails when Mercury comes to the stable where Viv works.  Her ambitions for Mercury, with herself as the rider, crescendos into an unhealthy obsession.  In fact, obsession is not even a strong enough word.  Viv’s passion for Mercury is more like an addiction.  I devoured this book.  The author drops a few too many broad hints of major trouble on the horizon, but she managed the suspense level really well with good pacing and excellent writing.  A moral dilemma eventually develops for Donald, and that, too, provided motivation for me to keep reading when I should have been doing other things.  Viv, on the other hand, is a somewhat one-dimensional character.  She may love her children, but her love of Mercury trumps everything else.  Donald’s biggest failing seems to be inertia, and he seems to be blind at times to what is going on with Viv.  Ironically, he is an optometrist, but his friend Jack, who manages to hide his blindness from his girlfriend initially, has better vision than Donald when it comes to a person’s true character.