Charles Sheffield 
My Brother’s Keeper [EPUB ebook] 

Lionel Salkind was glad for a visit from his twin brother Leo; the two rarely saw each other. Tehn their plane went down, and as he lost consciousness Lionel knew that Leo was dead. Lionel woke up in a hospital, to the eerie news that he owed his life to bits of his dead brother’s brain transplanted into his own skull. It was a very experimental operation, but it seemed to have been successful. And then the memories began. Terrifying snatches of memory that had no place in Lionel’s own quiet past. Leo’s memories, calling him somewhere to finish a job half-done – a dangerous job whose importance to the world Lionel had not even begun to grasp.