Fascinating how a simple word choice raises so many issues.
I think it's a perfectly valid subject for discussion, and the discussion here has certainly been respectful.
In the case of this book, using "girl" was appropriate for the various reasons suggested: it was an actual headline, and throughout the book the writer used actual news articles; it is "catchy" (alliterative with "gun"); and it captures the liveliness and humor of the book itself. It didn't bother me at all, and I am quick to react to what I perceive as sexism.
I think the use of "girl" is undergoing change in our culture, rightly so. But that doesn't have to mean that it only is used to for female children. There is the popular expression, "You go, girl!" as a mark of enthusiastic approval or admiration for something a feisty woman has said or done. An ironic tribute to female strength. In that sense, the word "girl" fits the title of Gone Girl by Gillian Flynn, but not The Girl on the Train. Still, it doesn't bother me much, given that it's only a book title; it would be much more offensive if it was a daily practice by a contemporary newspaper or on a TV commercial, and it is always offensive in social relations. Men obviously use "boy" as an insult to other men, often in a racial context; it clearly expresses that the "boy" or "girl" is less mature, educated, of lesser stature, inferior. But there IS a very recent novel using "boy" to refer to a man, Best Boy by Eli Gottlieb (2015). The nonfiction book The Boys in the Boat is another recent example. The Burgess Boys by Elizabeth Strout, A Suitable Boy by Vikram Seth... None refer to male children and none use the term as a put-down or negative. I conclude that using boy or girl in a title to refer to adults is not de facto a mark of unconscious sexism. It could be a mark of laziness or lack of originality, however.
Same with wife. Totally fits The Aviator's Wife, but too many wives in too many titles suggests too many poor marketing decisions to me.