“The subtitle of Mary F. Pols’ Accidentally on Purpose: A One-Night Stand, My Unplanned Parenthood, and Loving the Best Mistake I Ever Made (Ecco) would seem to give away the story, but read on anyway: It’s funny, intimate, wise, and real, and we get to root for forty­something Mary, her twentysomething co parent, Matt, and their little son, Dolan, now four.