The Vineyards of Champagne by Juliet Blackwell

 The novel bounces between the stories of newly widowed American Roselyn and newly engaged French girl Lucie.

Roselyn lives in current-day Napa Valley, working as a rep for a wine company and Lucie lives in the Champagne region of France during WWI.  Their stories intersect when Roselyn meets an outgoing Australian named Emma on a business trip to France. Emma is researching letters she's gathered between Lucie and a soldier named Emile. Roselyn becomes intrigued by the letters and wants to help Emma find out what happened to Lucie and Emile. At the same time, Roselyn is learning how to live again after the untimely death of her beloved husband, Dash.