Millionaire's Reluctant Mistress by Carole Mortimer

When Alejandro Santiago discovers he has a young son, his money and power mean he easily wins custody from the boy’s aunt, Brynne Sullivan. But he hasn’t counted on Brynne being so feisty, passionate — and beautiful.

He agrees to her plea for a month’s visit. There’s just one problem for Brynne: the Mediterranean millionaire is not just arrogant — he’s also infuriatingly gorgeous! As Brynne reluctantly gives in to the desire simmering between them, she knows that to Alejandro, she’s only a conveniently short-lived affair…