Jack Skowsays that Larry McMurtry's shaggy new novel "Dead Man's Walk" (Simon & Schuster; 477 pages; $26) passes the all-important "Call Me Ishmael" test. Its first line is "Matilda Jane Roberts was naked as the air." After that start, the narration wafts aloft into further elegant absurdity. If "Dead Man's Walk" were not a prequel to McMurtry's classic "Lonesome Dove," it would be worth only glancing notice. "As things are, it is a satisfactory foothill, with the grand old mountain in view. Still, by rough calculation, there are about three decades of imprudence and dusty commotion to get through before the main characters Gus McCrae and Woodrow Call become the leathery, intermittently shrewd men of 'Lonesome Dove.' The prequel hasn't really completed its job. Obviously a postprequel or two is required before the central novel can decently begin."