First, most of the reviews for this book appear to be for Dan Brown's The Da Vinci Code, which is incorrect. This book is a 60-page response that attempts to convince readers that Dan Brown's work of fiction is not based on as much fact as fans would like to believe.
Regardless of what you want to believe about Jesus and Mary and the their potential offspring, Garry Williams' The Da Vinci Code comes across as a weak argument. I felt like Williams cherry-picked a couple of minor points in Brown's work of fiction to go "ah-ha! you got it wrong!", and then proceeded to claim that the rest must be bunk too. Several of his claims seem to go back on themselves or try to connect unrelated bits of information. For example, Brown claimed there were more Gospels of the Bible, but the Bible says there weren't, so there weren't. (I'm not a Bible scholar, so I have no idea - but Williams would be more convincing if he had actual outside evidence.) Brown said men in Jesus's time were very unlikely to be unmarried, but Williams says one group (tribe?) did have a lot of holy men who didn't marry, so Jesus could have also been unmarried - but doesn't say that Jesus was in this tribe, or that this practice was known to occur in other tribes.
The second chapter is focused on convincing readers that the Gospels were written and copied over and over for two millennia, near perfectly. While I appreciate the argument that oral traditions were more important in that time and could have been passed down for generations with accuracy... Williams fails to even acknowledge that people might have intentionally changed the stories! We know people rewrite history - whether to suit their own purposes, or just to present someone in a better light - and I find it near impossible to believe these documents made it to today without any embellishment or editing.