When it comes to success stories, used-book stores are like three-legged thoroughbreds -- it's best not to put money on them, yet it's impossible not to cheer them on. So it goes with River City Books. Last December, Carrie Pacey opened the store at the entryway to the River Market, just across the Walnut Street bridge from downtown. With the help of her husband, Todd, Pacey has filled the small shop with high-quality paperbacks that are rarely priced at more than $10 and usually go for half that. Kansas City's budget-minded book lovers would be foolish not to keep River City Books in business. Plus, the Paceys are expecting a baby. Not that you should be influenced by that, but still.