Lady Killer by Lisa Scottoline.  
Mary DiNunzio is a trademark Lisa Scottoline heroine.  She's strong, she's smart, and she's got plenty of attitude. In recent years, she's become a big-time business-getter at Rosato & Associates, but the last person she expects to walk into her office one morning  in mile-high stilettos is super sexy Trish Gambone, her high school rival. Back then, while Mary was becoming the straight-A president of the Latin Club and Most Likely to Achieve Sainthood, Trish was the head Mean Girl, who flunked religion and excelled at smoking in the bathroom. As  it turns out, however, Trish's life has taken a horrifying turn. She's terrified of her live-in boyfriend, who's an abusive, gun-toting drug dealer for the South Philly mob. There's only one problem.