If you're in Kansas City, and can take a break from the ten zillion must-eat restaurants, Spivey's Books is a destination bookstore. It is a rambling, creaky building with a vast basement, sprawling ground floor and two distinct second floors (seriously bizarre layout).
The hardback SF/F collection isn't great, but there's a huge collection of vintage paperbacks and a mystery section that goes for rooms and rooms. It is also really easy to get lost in the ephemera - postcards, posters (from 1960s blacklights to movie posters to bizarre European stuff I don't understand), prints, clippings, magazines, bookplates, etc... Everywhere you turn, there's another box of interesting 'stuff' that invites discovery.
The prices are also very, very reasonable (especially if you're used to London). It becomes very easy to wander up and down creaky stairs for the better part of a day...
