My Brother's Keeper: Charles Sheffield

Lionel Salkind was glad for a visit from his twin brother Leo - they rarely saw each other. Then their plane went down, and as he lost consciousness Lionel knew that Leo was dead. Lionel woke in hospital, to receive the eerie news that he owed his life to parts of his dead brother's brain having been transplanted into his own skull. A very experimental operation but a successful one. But then the memories began. Leo's terrifying memories, calling Lionel to finish a job half done, a dangerous job whose importance to the world Lionel could not even begin to grasp.