It's also a classic horror/adventure story and is a great page-turner. Ultimately however the two narrative strands fall apart, and it's at this point that I began to feel that the story was all a bit silly and rather a waste of time. I think that the true problem is that Arturo Perez-Reverte has tried to be a bit too clever. A greater focus on one or other of the threads would probably have produced a more satisfactory, if perhaps less memorable novel. Interestingly in Roman Polanski's adaptation of the novel (produced as The ninth gate in 1999), the script-writers did focus much more strongly on the horror thread, and, I think, produced a much stronger narrative as a result.
A compelling read, but ultimately rather unsatisfactory.