**Mr. Costain's best book of fiction, and one that will put out a strong bid to rival The Robe in success with that market. The subject is an illusive one that has been approached tentatively through the centuries -- the mystery of the holy grail. As Mr. Costain conceives it, he visualizes the cup used by Jesus at the Last Supper in the hands of Joseph of Arimathea, an ancient and godly man who seeks a way in which the holy cup can be preserved. Through an agent he locates a gifted silver worker, an artist who has been falsely enslaved; he frees him, brings him from Antioch to Jerusalem, commissions him to make the container for the cup -- and thereby thrusts on young Basil a strange and dangerous mission. Many adventures, some that might have been debasing, many that brought him increase of power and deepened vision and spirituality, came his way. His marriage, as Joseph lay dying, to Joseph's granddaughter, Deborra, to save for the Christians some of Joseph's wealth; their escape from the predatory Aaron and zealots who sought to strangle the new faith; the mad ride across the desert; the verdict of the courts -- all these make a rich tale of adventure. But cutting deeper is the spiritual adventure, as Basil meets and is influenced by Luke and Paul, John and Peter- and many lesser figures, martyrs to their faith. The story is told with Mr. Costain's accustomed skill in recreating the richness of detail in background, period, setting- from Jerusalem to Antioch to Nero's court in Rome. Occasional pandering to modern taste weakens the fundamental values, perhaps, but insures the larger market. A sure best seller.