Back when I reviewed their first two books (not sure if I ever reviewed the third, I don't think so) on wotmania a few years ago, the blurbs made a big fuss about how the Vatican had prevented the books from being released, or in any case being widely sold, in their original version. Perhaps that was exaggerated even then, I never really checked. The Dutch translations do seem to have been among the earliest released, though - at least in Secretum's case, the Dutch edition was the first to be released anywhere, in 2005.

I just refreshed my memory on what the third was about, and based on that, I recall thinking that the plot was merely okay and the concept was getting a bit old, but it was worth it for the very interesting description of early 18th century Vienna. And the "reality" of their historical revelations is somewhat in doubt, I'm afraid... though whether that is because historians are little inclined to accept evidence presented in such a way, or because their case is actually not credible, I couldn't tell you.