To be completely accurate, the title of this book should read "Rosslyn, and the Grail". Because there is actually very little in this book that is about both Rosslyn AND the grail.
What we do have here is a great deal about Scottish myths and legends, quite a lot of which relates to Rosslyn, and a little of which relates to the grail. Moreover, a great deal of what concerns Rosslyn is really about Rosslyn CASTLE rather than Rosslyn CHAPEL. The promised revelation of the "secrets" of Rosslyn Chapel is contained in the 16 pages of the book's fourteenth and final chapter (there are a further 80 pages of appendices), and it really doesn't amount to much of any consequence.
I appreciate that the primary purpose of the book is to set the record straight regarding various distorted accounts about Rosslyn, the holy grail, King Arthur, etc., and the authors have done a reasonably good job in this regard. It must also be said, however, that the only real link between King Arthur and the holy grail is found in Robert De Boron's three part 'romance' - "Merlin and the Grail" - still available from Amazon.
(This is also the main source for the idea - crucial to the conspiracy theorists' ideas about Leonardo's painting of the Last Supper - that the grail was a "chalice". In Chretian de Troyes' story "Perceval" it was a dish, and in Wolfram von Eschenbach's story of "Parzival" it was a stone.)
On this basis much of the supposedly "historical" material is actually about the legends surrounding King Arthur and his knights, which is entertaining enough, if you like that sort of thing, but not pasrticularly useful as anything but a commentary on one very limited area of the literature of the 12-13th century.
In short, if you're looking for a very general guide to Scottish folklore, etc., you might well enjoy this book. But if you're looking for an in depth guide to "The truth behind The Da Vinci Code", as implied at the top of the front cover, then in my opinion this will be pretty much a waste of money.
(I would have given the book 3 or even 4 stars if it was presented as what it really is.)