There are many legends about the Holy Grail. This book is just another possible explanation as to what happened to the Holy Grail or the cup used by the Lord during the Passover.
In this book, the Holy Grail was made by the Lord himself in Joseph's carpenter shop in Nazareth. The wood used was lignum vitae, which translated that means the "wood of life." There is then the connection between the wood of life and the tree of knowledge of good and evil in Genesis.
Jesus etches a lamb on the side of the cup as a teenage boy, not knowing that later he would be the lamb to be sacrificed. His mother Mary takes possession of the Holy Grail after his death. In turn, she passes it on to different people.
Meanwhile, the Holy Grail is used to perform some miracles. Eventually, it is lost in a storm in the Tyrrhenian Sea while in transit to Rome from Corsica.