Deceptions by Rebecca Frayn

Julian, an art authenticator, lives with his girlfriend, Annie, and her two children, Dan and Rachel. Soon after Julian and Annie announce that they plan to get married, Dan disappears. All the time he's missing, Annie keeps his room exactly the same and holds out hope that he'll return. After three years, a call comes from Glasgow saying that Dan has been found and is on his way back to London. By this time, Julian and Annie are living apart, mostly because Julian has wanted Annie to move forward with her life--he's seen firsthand how Dan's disappearance has made Annie so depressed. The Dan that returns home is very different from the boy that left. Now fifteen, Dan looks and acts differently and has no memory of where he was all that time--or even memories of growing up. Annie doesn't notify the authorities of Dan's return and refuses to press the issue of where he's been, which deepens the rift between her and Julian. But for Julian and the reader the question remains, where was Dan...and is it really Dan at all? Deceptions is similar to the novels of Ruth Rendell, Tana French, and Sophie Hannah.