The Dark Rose by Erin Kelly

Louisa, nearing forty, works in Warwickshire, restoring the gardens of Kelstice Lodge. She's still haunted, however, by an event twenty years ago involving her then-boyfriend, Adam. When Paul, not yet 20, comes to work at Kelstice Lodge, Louisa is stunned by his resemblance to Adam. Paul, it turns out, is hiding from his own past. Paul has given evidence against his best friend, Daniel, implicating him in a murder. He worries what would happen if Daniel or Daniel's father, Carl, find out where he's living. Combining the present story of Louisa and Paul along with slowly revealing their pasts, Kelly has written a novel of psychological suspense that's hard to put down once you're into it. For readers who enjoy Ruth Rendell and Tana French.