Book excerpt: In Falling Through the Music, his fifth major book of poetry, Mark Halperin gives us consolation, guidance, and companionship while delivering an accomplished meditation on the first real glimpses of the limits on a life. Displaying an agility of formal invention--he moves easily from a Whitmanesque and witty litany to rhymed quatrains--Halperin deftly melds technique to theme. As in "Someone Pausing, he is able to place us in the mind of someone--any one of us--who has stood on an island in the street, fully attentive and present, knowing nothing stays, not even the observer.