Letters to the Lost by Brigid Kemmerer

Letters to Letters is a novel by New York Times bestselling author Brigid Kemmerer. It's about Juliet, who is drowning in grief after her mother's death. Declan is trying to escape the demons of his past. Writing letters to her mother is the only way Juliet can cope with her loss, but when Declan finds one of her letters and writes back, it sparks an anonymous correspondence.