Snow Angel by Mary Balogh

A sudden snowstorm strands Rosamund Hunter and Justin Halliday, Earl of Wetherby, at a hunting lodge in the country. Despite their initial embarrassment over the fact that they are strangers, they soon relax and enjoy a magical three days of friendship, fun, and passion. They part reluctantly, expecting never to meet again. She is on her way, after a year of mourning for her late husband, to rejoin her family for the celebration of the long-awaited betrothal of her niece. He is on his way to honor a match arranged long ago by his mother and Rosamund’s and recently made more formal when he made a definite offer to her brother and guardian.

By the time Rosamund and Justin realize that they are bound for the same place, it is too late for him to withdraw honorably from his betrothal and far too late for either of them to avoid heartache.