The dark, poetic music of Leonard Cohen should be listed on the table of periodic elements — when you discover it, it suddenly seems as necessary as oxygen. This collection spans his thirty-five-year career, and it doesn’t leave out much. Originally a Canadian poet and country-music fan, Cohen moved to New York in the late Sixties and became a folk singer. Now a Buddhist, Cohen has always been concerned with the sanctity of real life and the dangers of real love. The startling “Hallelujah,” from 1984, is a majestic example of the former, and there are too many examples of the latter to count, from the darkly sly “I’m Your Man” (1988) to the lost and forsaken “Famous Blue Raincoat” (1970). One could complain about the absence of various wonders, such as his early, way-bleak “The Butcher,” or object to the inclusion of four songs from Cohen’s most recent album, but that would be quibbling. These are literate songs that require time, careful attention and the sort of contemplation we normally reserve for life lessons.