The book is called The Holy Grail: The History of a Legend, by Richard Barber. Being a study of the real Grail literature, it says nothing about Mary Magdalen, but has lots of references to the tradition that Joseph of Arimathea pppreserved the actual blood of Jesus (gathered while he prepared the body for entombment) as a relic, preserved it in an object that may have been either the cup from the last Passover or the cup Jesus was offered on the cross.

It quotes extensively from the early Grail writers - Chretien de Troyes, and Wolfram von Eschenbach in particular, and follows the development of the Grail legend in literature right up to modern works such as T.S. Eliot's The Waste Land and films such as Excalibur and The Fisher King.

The first printing was before Brown published his book, but the paperback edition, which I have, does include an afterword of a couple of pages saying how silly the whole Da vinci thing is.