I have mixed feelings about this book. On the one hand, I enjoyed the main character, Sister Fidelma. She's smart and inquisitive and strong-willed and doesn't put up with crap from anyone. She's a little too perfect at times -- I prefer characters to at least have some flaws -- but still, she was engaging and I ended the book interested in reading more about her. I also liked the main plot: the murder mystery had a nice "cozy" feel to it, with enough danger to add a bit of tension but not enough to slip into horror or gore. I also enjoyed the interactions between Fidelma and the monk assigned to help her, and the hints at a possible romance (in the days before celibacy in their respective orders). The setting was definitely interesting, and gave lots of insight into the society of the time. The main problem I had with the book was the amount of "historical" that I got in my "historical fiction" novel.
The story takes place during a famous debate between representatives of the Celtic and Roman churches in AD 664. How this debate progresses is treated not just as a backdrop to the main storyline about Fidelma solving the mystery, but as a major plot line on its own, which took up a good deal of time in the early chapters to establish, and was revisited throughout the book. I didn't keep close track, but it felt as though at least as much time was spent listing the names of all the people who were there (and sometimes, the names of people they were related to, who weren't there), outlining the various positions being argued, and discussing the effects of these religious issues on the Celtic people as on the main story. It's an interesting debate, and I wouldn't mind reading a history about what actually happened there. But this narrative is obviously highly fictionalized, so I couldn’t view it as educational because I couldn't tell which parts of the debate-plot were true and which were made up for this book. Also, this meant that only half the book remained to flesh out the main story, about Fidelma and the mystery -- time was used up for the debate that could have been spent making the characters and their dilemmas more three-dimensional. The debate itself had no discernible impact on the main story, either. It was an event that brought together people from various factions, and perhaps the murder victim was killed to keep her from speaking there -- but that could be true of any number of events. Also, even if I accept the idea of the debate as a reasonable plot line of its own, the exposition dumps in the early chapters felt excessive, especially the long lists of names, precious few of which turned out to be relevant. It smacked of a writer who had done a ton of research and wanted to prove it, and couldn’t stand leaving out a single detail.
A related gripe was what I saw as an overuse of Latin words. I usually appreciate getting lots of cultural details, and for the most part, this book does a great job of that. And if a language uses a term for a cultural concept we don't have in English, it makes sense to use the foreign word -- like when he uses an Irish word for the specific kind of legal role Sister Fidelma holds. But it distracted me and slowed my reading to see "officium," "cubiculum," "sacrarium," etc. in italics on so many pages, which was unnecessary since these terms didn't convey anything different than office, cubicle, sanctuary, etc.
Overall, I liked the character of Sister Fidelma, what I got to see of her, and would like to think that she's developed more in future installments. I'm definitely curious about what happens, if anything, between her and Brother Eadulf. I thought the book was strongest when it stopped telling us all about the history and culture of these people and focused on showing us how people from the different cultures clashed because of their disparate expectations and norms. If there's more of that in later books, and less of a slog through questionable history, I'd be interested in reading them.