 His best-known work, "Letter to an Imaginary Friend," was published in sections between 1957 and 1985 and as a single poem in 1997 by Copper Canyon Press.Best of all, Letter to an Imaginary Friend licks its fingers and burps at the table.Polite it is not--and the better for it when McGrath turns from his populist vitriol to what may be his most abiding talent