The Billionaire Next Door by Christina Tetreault

Curt Sherbrooke has two passions, renovating old homes and writing novels. When he leaves the financial world and the city behind, his plan is simple: indulge both while keeping his true identity as a member of America's wealthiest family a secret from his neighbors. Soon Taylor Walker, the sexy DEA agent living next door, and her niece have him reconsidering his plan.
Taylor Walker doesn't believe she needs a man to complete her life. She's happy raising her niece and taking drug dealers off the street. If Mr. Right happens to come along someday, that would be wonderful. If not, she's not going to lose any sleep over it. Then she meets Curt, her new neighbor.
Soon Curt and Taylor appear headed for their own happily ever after. At least until Taylor's older sister returns and tries to capitalize on Taylor's newfound love.