This Regency set Historical is the debut from Diane Perkins and was the winner of the Romance Writers of America 2003 Golden Heart for Long Historical.
From the back cover:
The Mysterious Miss M is a living male fantasy - alluring, sensual, masked. But when Lord Deviln Steele finds himself responsible for her - and her child - he comes to know the real Maddy: the loving, passionate woman who drives away the nightmares of the Waterloo battlefield.
But the aristocratic soldier can’t support his new family. He ll only inherit his fortune on marriage to a suitable lady - and Maddy is far from suitable. With the dangers of London’s underworld closing in, how can he protect the woman he has come to love?