Remember Me?, by Sophie Kinsella.
I loved, loved, loved Sophie Kinsella's The Undomestic Goddess, about a lawyer who is mistaken for a housekeeper. I enjoyed her Shopaholic series, but not as much - I wasn't as fond of her ditzy, no self-control, overspending heroine. I think that I like Kinsella's books because they are the right balance between business/career (Kinsella was a financial journalist before turning to witting) and romance (which, as a married woman, I've officially sworn off). Remember Me has got to be one of her best yet. Because of the heroine's amnesia, the story has a real element of mystery to it - how did her life change so much during the three years that she can't remember?