Haunters by Thomas Taylor
Eddie, Adam and David have the same gift. Separated by generations, they are linked by their ability to time-travel. Using their dreams, they can appear like ghosts, wherever and whenever they want. The first is the genius who discovers dreamwalking. The second is a Haunter, a dream-terrorist, determined to change history for his own ends. The last is the novice dreamwalker who must battle to save his family, and himself, from oblivion.