Emily Grayson is a similar author. Grayson writes in a slightly more dramatic style about romantic relationships with endings that are as emotional as Sparks' but less bittersweet.
Another author is James Michael Pratt. His first three books, The Last Valentine, The Lighthouse Keeper and Ticket Home, use the dramatic backdrop of World War II to tell the stories of couples who are separated by war but joined at the heart. Pick up The Lighthouse Keeper first.
K. C. McKinnon's Candles on Bay Street is supposed to be another good one.
