Anyone who remembers tackling T.S. Eliot’s “The Waste Land” in school, trying to tease meaning from the dense modernist poem, probably knows just what Matthew Zapruder means when he takes issue with poetry’s unfortunate reputation for being “deliberately difficult.” A poet today can seem like a practitioner of some arcane, inscrutable art form, and thus many people never return to the genre — unless they’re in need of a verse for a wedding or eulogy. In his penetrating yet refreshingly straightforward new book “Why Poetry” (Ecco; $24.