Galway crime master Ken Bruen is back with a new thriller that is dark and hard-boiled even by the very high standards he has set in previous novels such as "The Guards." Just don't expect this book to be endorsed by either the Irish or New York police benevolent associations. Bruen's latest, "Once Were Cops," follows an unhinged Irish cop named Matthew Patrick O'Shea who, in typically underhanded Bruen fashion, finagles his way to New York to spend a year working with the NYPD. Once there, "Shea" teams up with an equally disturbed partner, Kurt Browski, who, when not maiming and thieving, is being investigated by Internal Affairs for his close association with known organized-crime figures. Shea's lies and Browski's scheming are on a collision course, setting up a chilling conclusion to "Once Were Cops." Bruen has always explored the deepest, darkest themes of crime and punishment among cops and criminals. But it is Bruen's style in "Once Were Cops" which stands out. It is perhaps his most unsettling performance yet, making even Mickey Spillane's prose look tame. "Took his legs out with the hurley. Swoosh. I love that sound," reads one typically gruesome passage. Bruen, best known for his Jack Taylor series, has written another winner for his loyal fans. ($22.95/294 pages / St. Martin's Minotaur)