A Poisoning at Castle Gloaming

After a two-day train journey, peripatetic dressmaker Jemima Flowerday makes the final approach to her new job on foot, following the canal to Castle Gloaming, which stands in the rugged North Wales countryside overlooking the foaming River Dee.

Jemima has come to clothe Mrs Cornelia van Doorn, a wealthy South African widow who has leased the castle from its impoverished aristocratic owner, intending to launch herself and her young stepdaughter, Honor, into London society. But on her arrival she’s told she’s actually been summoned to investigate the disappearance of the girl. Even in a place still haunted by mystery and magic, the seventeen-year-old can’t have melted into thin air…

"A Poisoning at Castle Gloaming" was written by Kay Blythe.

Kay Blythe, who also writes as Natalie Meg Evans, is an award-winning historical author
on both sides of the Atlantic, having reached the New York Times top 100 list with her
debut novel, The Dress Thief. Writing crime as Kay Blythe fulfils a long-held ambition. Her
dressmaker-sleuth, Jemima Flowerday, follows in the tradition of clever women set free by
the social upheaval of the years after the First World War. Jemima combines her skills as
a dressmaker and sleuth to solve crime in the crumbling stately homes of Britain.

Yes, "A Poisoning at Castle Gloaming" is part of the "Jemima Flowerday" series. It's book 2 in the series.