[The temple] might just as well have been dedicated to the Breton sea-queen Dahut as to Neptune. This Gray supposed to be the reason the Professor had described Duke Gaël's refurbishments as insufficiently ambitious; a man more convinced of the superiority of Roman worship, law, and custom he had never yet encountered. [loc. 638]
A fascinating alternate history, set in an early nineteenth century comparable to the Regency period, but with no Napoleon and with Henry XII on the throne of Britain -- a Britain which does not include Alba or Eire, but comprises the provinces of Cymru, England, Kernow, Normandie, Maine and Breizh. Magic may be studied at Merlin College in Oxford, where Graham ('Gray') Marshall is an undergraduate, tutored by Professor Callender, Regius Professor of Magickal Theory.
A fracas between town and gown sends Gray into exile at the Breizh home of his tutor, where he makes the acquaintance of the Professor's daughters: elegant Amelia the eldest, mutinous Joanna the youngest, and Sophia, the middle sister, who tries to avoid notice. This chameleon quality is so effective that Gray suspects her of having magickal talents, but Sophie protests that her father has always told her she has no magick.
The Winter Queen is in the same general territory as Sorcery and Cecilia (Wrede and Stevermer: perhaps the earliest of the Regency-romance-with-magic genre), but it's more Gothic. It lacks several of the usual trappings of the pseudo-Regency romance: Sophie is not especially interested in fashion (though Amelia is); there are dastardly conspiracies; there is a shadowy plot involving the Midnight Queen, the Breton second wife of King Henry; the mysterious Mrs Wallis, Sophie's guardian, knows more than she says; and of course there is magick, and counter-magick.
The plot is byzantine, the characters (especially the women) have distinct and rounded personalities: but what I found most interesting was the worldbuilding. This is a Britain with Romano-Celtic, rather than Judeo-Christian, roots: Ivor (not Isaac) Newton's Principia alchemica is a standard text; Oxford boasts temples to Minerva and Apollo, though Sophie's offering is left at the temple of Mercury and Epona; a knowledge of Cymric is essential for any scholar; and Joanna struggles to comprehend the monotheistic Judæi's insistence on a single deity. 'How peculiar. He must be terribly busy.'
This enjoyable novel is the first in a trilogy: I expect I'll read the other two, not least to find out more about this particular variant of history.