Opposites do more than just attract in TONI ALEO's latest Nashville Assassins novel about a very bad boy and the good girl he can't resist.   The instant Piper Allen sees Erik Titov, she wants him--wants his rock-hard body, sure, but the strength and mystery that lie behind that superstar hockey jock demeanor, too. So when he sidles up to her at a bar and slinks his arm around her waist, she's lost. What follows is the wildest night of her life . . . followed by inevitable heartbreak the next morning. And then, a few weeks later, a very big surprise: two blue lines on a pregnancy test.   Only a check to the head could make Erik fall for a nice girl like Piper. But since their crazy-sexy night together, he's been trying to forget about her alluring body by falling into bed with every woman in Nashville, and it's not working.