THE ESSENTIAL LEONARD COHEN manages the considerable task of documenting the career of a man many consider one of the finest songwriters in the English language. Easily on par with Bob Dylan and Paul Simon in terms of sheer poetic power, Canadian troubadour Cohen started out, appropriately enough, as a prose poet before beginning a recording career in the late 1960s. His early albums, amply represented here, found him accompanied chiefly by his own nylon-string guitar, delivering dark, but often romantic, reveries atop folk-based song structures.