I've read nearly all of Marillier's books, loving the Sevenwaters trilogy and having mixed feelings about the others. I have not read Wolfskin, the prequel to Foxmask, but read this one anyway because so many of Marillier's fans adore it. My reaction: mixed. This book is a good example of Marillier's storytelling skills; it bears more resemblance to the original Sevenwaters trilogy than to the Sevenwaters follow-ups or Heart's Blood, which felt much more commercial (faster-paced, less lyrical, more conventional). Still, it's not one I'll re-read.
Foxmask follows two main characters: Thorvald, who embarks on a quest to a distant island searching for the father he never met, and Creidhe, who follows Thorvald out of infatuation but grows up along the way. Their parallel storylines are set up well, with a slow-paced, immersive start and gradually increasing tension up to the major events of the last third of the book. Creidhe is a decent but not spectacular heroine, and Thorvald was more interesting than I expected; in all the other Marillier books I've read the male lead was also the love interest, but Marillier chose wisely in not casting Thorvald in that role, allowing him to be selfish and pursue his own interests. Ultimately I couldn't tell whether we were meant to like him or not, and was glad of it. But Somerled was disappointing; from what other characters said about him, I expected someone with more drive and conviction, and was sorry to see he'd turned into such a conventional character.
A few more words on the plot and characters. There are several mysteries and puzzles, to which the reader is likely to guess the answers before the characters; but, fortunately (and unlike in the more recent books) Foxmask doesn't depend on mysteries, and characters figure things out as they go rather than wandering about clueless throughout. However, they did frustrate me in that I could often think of several better solutions to their problems than the ones they chose, solutions which tended not to occur to anyone.
And then the romance (which is the reason I keep reading Marillier books after all). This one didn't capture my emotions. It wasn't poorly written; I understood the characters' attraction to each other and liked that they shared many of the same values. But their journey from initial meeting to the consummation of their love happened very quickly and easily (as it had to given the structure of the book and the characters' situations), and for me to care about a romance it generally needs to take longer than that, and have more complications and obstacles. Just because it's not my thing doesn't make it bad, and the romance here is secondary to the larger story of the islanders' plight and the main characters' growth and development, but still I was disappointed.
Finally, there's that larger story; it's difficult to discuss without giving anything away, so some minor spoilers follow. This story felt a bit mythological, although I'm not sure where it came from. I really liked that Marillier managed to make the two major male characters enemies based on their circumstances without making either "evil." The characters face a variety of moral dilemmas, mostly revolving around the question of when it's acceptable to sacrifice the few for the sake of the many (or vice versa). While I would have liked the book to be a bit less conventional in its answers, the end certainly leaves room for discussion about who was in the right and how the characters should have handled their situations, and it isn't all clear-cut. The drawback to all of this is that the situation itself is contrived in a variety of ways, the characters hemmed in by a bunch of arbitrary rules. (The strangest: There's an extremely remote tribe with no blondes, which requires a blonde woman to conceive its seer. How do they typically manage this?) The book has a fairytale resonance that kept this from becoming too annoying, but the setup is not exactly logical.
What really irked me, though, were the modern beliefs of the characters. They're pro-democracy. They're anti-arranged marriage. I have a sneaking suspicion that they also support paternity leave and free universal health care. The phrase "due process of law" is used. (As an American, I'm proud to see phrases from my country's constitution pop up even when an Australian author writes about people living on the Orkney and Faroe Islands over 1000 years ago. But it makes no sense at all, given that these people have neither written laws nor a formal judicial system.) I know Marillier writes feel-good historical fantasy, but this was a bit much.
So this isn't a bad book; I see how it could work for a different reader. The writing style is decent and there's a couple of good coming-of-age stories in there. But readers new to Marillier should be buying Daughter of the Forest, not Foxmask.