In Forbidden Territory by Shawna Delacorte

Filling in as a guide and companion for his best friend’s kid sister was cramping Tyler Farrell’s bachelor lifestyle. At least, until a drop-dead gorgeous, very grown-up Angie melted his bones with one searing kiss. She was supposed to be off-limits...but Tyler liked playing with fire. Angie wanted to prove she wasn’t a child anymore. And what better way to do so than a no-strings affair with Tyler — the perfect love ’em and leave ’em guy? Except that the closer she got to Tyler, the hotter the attraction burned and the harder she fell in love. Somehow she had to convince commitment-phobic Tyler that she was his to claim — permanently!