“The Things They Carried,” a searing novel about grunts in Vietnam, is American author Tim O’Brien’s legacy and crutch. His latest, “Dad’s Maybe Book,” seems far different: a scrapbook of writings to his two sons from early on, arising from the yearning of an old father to leave a legacy. Jottings and serious essays and emotional letters, it’s a pleasingly complex and diverse package. The tone is that from one adult to another, and O’Brien is a humble, if pernickety writer. He writes about special books, about the war he was in, about his parents, about being an amateur magician, about the sons being addressed. In the end, he’s writing words of love and that emotion suffuses the pages. Some of the pages can sound like peripheral asides, and the overall arc carries little heft beyond the letters themselves, but I enjoyed a wise writer’s company. And I was amazed to discover the persistent shadow of the Vietnam War through “Dad’s Maybe Book.” Tim O’Brien leaves a war novel as his chief legacy and now passes it on to his offspring.