The Dragon Quartet, Volume II: The Book of Fire, The Book of Air

Here's an exciting fantasy tetralogy with all the right ingredients: four elemental dragons (Earth, Air, Fire, and Water), each with a human companion. This series takes us from medieval Europe to the distant future, as the world is caught in a war between the forces of greed and fanaticism, and the dragons and their guides and allies who seek to restore the proper natural balance to the planet.