The Wives by Tarryn Fisher

Polygamy and mental illness plus a very unreliable witness make for one heck of a roller coaster ride.
Thursday is happy in her "some days" marriage to Seth but the seed of doubt works its way into her brain and she becomes obsessed with the other wives. She looks for more information about them and begins to stalk them believing that Seth is abusive but is Thursday seeing the whole picture.  The first half of the book had me balancing the pros and cons of this very different type of marriage agreement and then the second half turned the tables.  A very fast-moving and clever plot draws you in and keeps you reading until you realize that you have been reading for hours and you don't want to put the book down.