Hunter (Bookshop by the Sea) opens this heartwarming romance with Queens, N.Y., western writer Sadie Goodwin learning that her publisher wants her to switch genres to romance. Luckily, her best friend offers Sadie the use of her mother's South Carolina beach duplex for a summer of writing. There, Sadie meets grouchy Sam Ford, the duplex's other tenant, and sets up a Little Free Library for the community. When someone leaves a book in the library containing a secret compartment hiding a beautiful engagement ring, Sadie's determined to get the book back to its proper owner--whoever the bookish proposal was meant for. To succeed, however, she'll need Sam to use his connections as a local. In exchange for helping, Sam asks Sadie to be his plus one at the wedding of his former girlfriend and his cousin, though he doesn't explain just how fraught the situation is. Working together leads Sam to warm to Sadie's endlessly cheerful nature and Sadie to see behind Sam's gruff exterior, but Sam struggles to open his heart to love after having been burned before. Hunter's charismatic and complex characters effortlessly propel the story. Readers won't want to put this down. Agent: Karen Solem, Spencerhill Associates. (Mar.)