check out works by classic composers [ie bach, beethoven, mozart, chopin, vivaldi, tchaikovksy, dvorak etc] or newer artists [i'm not very up to date on current composers, so i'll leave those for someone else] - stars of the lid, sigur ros, mogwai, etc
perhaps check out the OST for halo 3/other games [i'm not a fan of the series, but the OST for final fantasy are usually very good]
Martin O'Donnell & Michael Salvatori - Halo 3 Original Soundtrack (2-CD Set)