Lawhead has written some good books, and the first book in this series; the Iron Lance, was a promising start. The story mixes ancient and a more recent (early twenthieth century) storyline together, but the core of the book is the story of Duncan, the son of the redoubtable Crusader Murdo from the first volume of the Celtic Crusades.
Sadly, the Black Rood (the Holy Cross) does not live up to the standards of the first book. Murdo's story meanders across France and the holy land at a pedestrian pace, while the author describes a journey that does little to advance the story. We are told repeatedly about how much Murdo fears being executed the next day (the story purports to be his diary in Egyptian captivity), but as another reviewer noted, this quickly becomes irritating with several hundred pages yet to go. One would think that an author in fear of his life would hurry forward to the important events of his tale, but instead Duncan dwells on small unimportant details of his journey across France. This makes the story feel sadly pedestrian, and because we always know that Duncan is going to get through unscathed (he is telling the story, after all), suspense and tension are practically absent from the story.
The suspense of the reader might instead have been raised on behalf of Duncan's companions, but unfortunately here the book fails miserably. The companions are flat and 2-dimensional. Padraig is Duncan's conscience personified, his relationship with Sydonie mostly happens "outside the book" and Yordannis and Roupert remain cyphers. In general, characters turn up to help Duncan (usually with money or food) when needed, and disappear again just as easily when their purpose in the plot is served. The ending is a major deus ex machina; the big confrontation that might finally have tested our hero's character and personality (and show-cased the conflicts of interests between him and his friends) is simply resolved to allow our hero to earn his "trophy".
The modern storyline is not resolved yet; for that we must await the final book in the series. On the whole, this is not a book I would recommend to anyone who is not a serious Lawhead fan.