Divorce is in the literary air lately. Maggie Smith, whose poem “Good Bones” went viral in 2016, released a memoir last year about getting divorced after her husband couldn’t take her success; the nonfiction writer Leslie Jamison’s new book, Splinters, is about splitting up with her husband not long after their daughter was born; Ursula Parrott’s 1929 novel, Ex-Wife, was reissued last spring to a warm reception. It isn’t a surprise, then, to encounter a new release—This American Ex-Wife, by the journalist Lyz Lenz—that approaches divorce in a style that has all but taken over popular nonfiction directed at primarily female audiences: a light mix of history and social commentary that leans heavily on personal storytelling without quite turning into memoir. Common though it is, this hybrid form is tough to pull off. It can tempt writers to map their own experiences too neatly onto collective ones while also undermining the specificity and perspective that a good memoir needs. This American Ex-Wife suffers from both of these problems. Lenz’s impulse to generalize is so strong …