His most provocative and experimental work of the sixties, Beautiful Losers affirms Leonard Cohen as a visionary songwriter and novelist.     Beautiful Losers is a novel of stunning prose that is equal parts vulgar, rhapsodic, and viciously witty. At the center of the story are three individuals, united by their sexual obsessions and mutual fascination with Catherine Tekakwitha, a seventeenth-century Mohawk saint. A bereaved narrator reconstructs his relationship with his dec