You're taking the last plane out of Clarksville, and you forgot your book. And your gum, your bottled water and your earplugs. Not to worry: W. H. Smith keeps one of its six airport shops open all night, and it's conveniently located in the main terminal, right outside the security checkpoint that's the last to shut down for the night. In keeping with its 24/7 status, the shop stocks a wider variety of items than you'll find at its concourse outlets, including reading materials, snacks and travel necessities. Those landing at DIA in the wee hours, after their planes have been socked in by the weather in Omaha, might want to stop by and stock up on aspirin: They'll need some after they realize that the shuttles have quit running.