When my mother returned from a visit to the United States in 1960, she brought with her a book that I still possess. Its title was To Kill a Mockingbird. I also have a paperback version published in 1974. Blazoned across its ugly plain orange cover is the fact that the novel had by then sold 15 million copies. The fiftieth anniversary edition, published (not surprisingly) in 2010, claimed a total of 30 million copies. This claim might raise the hackles of those who think that literature is, or ought to be, a secret garden to the lock of whose entrance only the elect should have the key: but, as Dr. Johnson wrote in his otherwise not altogether laudatory biography of Thomas Gray, “In the character of his Elegy I rejoice to concur with the common reader; for...