“Gangbuster,” by Alan Prendergast. (Citadel Press) It’s hard to believe that in the 1920s, Denver was one of the country’s biggest crime centers. Under the heavy hand of a mug named Lou Blonger, dozens of grifters operated freely, fleecing dupes in intricate schemes. (Think “The Sting” without Robert Redford and Paul Newman.) The city was so corrupt that when newly elected District Attorney Phil Van Cise took office, he didn’t tell public officials or the police department about his plan to clean up Denver.