I’ve liked most of the books in this series, and this is one of the best. It really works as a mystery: Tremayne does a great job of leaving you enough clues to figure out the solution, but hides them well enough that you’re not likely to see the solution until about the time Fidelma does her summation.
The main defect in these novels is the long-windedness and excessive historical detail. Tremayne buries the reader in a pile of unfamiliar Irish legend, history, and geography, little of it germaine to the story line. Pro tip: when Eadulf acts confused about some place or person that he hasn’t heard of before and someone offers to tell him about, you can safely skip the next (lengthy) paragraph. On the other hand, pay close attention to the lineage and biography of even the most minor characters, because generally there will be a clue there that will help you unravel the plot.
Tremayne has managed to knock some of the rough edges off of Fidelma’s personality - a great improvement, in my opinion. In some of the early novels she was, quite frankly, a bitch - a very smart, capable, and determined bitch, but a bitch nonetheless. In the more recent novels she’s still quite determined, and somewhat irrascible, but is generally less abrasive than previously.
And the Eadulf character has improved, as well: he’s no longer the entirely clueless sort of Dr. Watson character that he was in the early novels. He manages to reason things out on his own for the most part, though he’s still eclipsed by Fidelma when it comes to the final reveal.
The only serious problem in this novel, and it’s a problem common to nearly all good whodunits, is the sheer implausibility of the chief investigators being in just the right place at just the right time for events to unfold under their noses. But, again as with nearly all good mystery novels, Tremayne makes it seem plausible at the time, so it’s only in retrospect, as you reconstruct the story in your memory, that you see how very unlikely the story is.