The Book That Matters Most By Wendy Smith, Special to The Washington Post Book groups have been a popular plot device in commercial fiction. It’s a handy way to get a group of people together, and they’ll always have something to talk about. Given the high-toned volumes and literary discussions featured in such best-sellers as “The Guernsey Literary and Potato Peel Pie Society” and “The Jane Austen Book Club,” it’s an amusing change of pace to meet a book group member like Ava, the protagonist of Ann Hood’s new novel.