The sixty poems selected for Pocket Book of Poetry span more than four centuries and some rank among the greatest works of literature in the English language. Many are popular favorites and several represent the best works written by their authors, among them William Shakespeare's sonnets, Samuel Taylor Coleridge's "Kubla Khan," John Keats' "Ode on a Grecian Urn," William Butler Yeats' "The Second Coming," and Robert Frost's "The Road Not Taken."   Although some of these poems share themes and verse forms, each is a unique work unto itself. All suggest a world much greater than can be encompassed in their words, and the way in which they transport the reader to that realm is a large part of the pleasure that they offer.   Pocket Book of Poetry is one of Barnes & Noble's Collectible Editions classics. Each volume features authoritative texts by the world's greatest authors in an exquisitely designed bonded-leather binding, with distinctive gilt edging.