At the age of 80, with almost 40 books behind him and nearly as many accumulated honors, Harold Bloom has written, in “The Anatomy of Influence,” a kind of summing-up — or, as he puts it in his distinctive idiom, mixing irony with histrionism, “my virtual swan song,” born of his urge “to say in one place most of what I have learned to think about how influence works in imaginative literature.”