Ava Bigtree, the new Scout FinchSwamplandia! By Karen Russell. Knopf; 336 pages; $24.95. Chatto & Windus, GBP12.99. Buy from Amazon.com, Amazon.co.ukCHILD narrators are often too precocious or too cute to work in novels. But when a story is capably carried on small shoulders—a Huck Finn, say, or a Scout Finch—the result can be a fine mix of guilelessness and something wistful. If all melancholic yarns essentially knit naivety with nostalgia, then a young hero can be a heady thing. ...