The Dragon King trilogy (composed of In the Hall of the Dragon King, The Warlords of Nin, and The Sword and the Flame), the Song of Albion trilogy (composed of The Paradise War, The Silver Hand, and The Endless Knot), The Pendragon Cycle (composed of Taliesin, Merlin, Arthur, Pendragon, Grail, and Avalon), the Bright Empires series (composed of The Skin Map, The Bone House, The Spirit Well, The Shadow Lamp, and The Fatal Tree), and Dream Thief, all written by Stephen R. Lawhead. I also recommend C. S. Lewis' Space trilogy, if you haven't read it. Oh, and Archives of Anthropos, by John White.