Emma Donoghue was a successful novelist long before her seventh novel, “Room,” eclipsed all her previous work and brought her international fame. If you read it, you’ll never forget 5-year-old Jack, who describes living his entire life with Ma in a backyard dungeon. We see their lives as an unspeakable ordeal of deprivation and rape, but Jack’s mother makes sure that her son sees their tiny cell as a world filled with wonder. With her new book, “Akin,” Donoghue returns to the story of a child and an adult trapped together. But the circumstances are far less bizarre, the constraints less intense. If “Room” was a horror novel laced with sweetness, “Akin” is a sweet novel laced with horror. It’s the story of a man learning late in life to expand his sense of family, to realize as never before who his kin are.