Gilman's Parallel The French King - Robert W. Boyer

Robin Peter Gilman has never seen proof of the Holy Grail. But all of that changes in Llangollen, Wales, when a fairy mysteriously appears in his life and shows him the existence of a parallel universe and a destiny he never could have imagined. He finds new friends and powerful allies. Nothing happens without reason, however. Robin is the future king of the two parallel worlds but so is Peter