Susanna, who wrote this book, is Canadian. She visited Eyemouth in the Scottish borders some yeas ago to research it. I live in the town and know the area quite well, also having authored two books that are about the place(s) and the people. Shadowy Horses lies in the 'romantic fiction' world, with a young lady looking for long term love; handsome young and older men and a touch of local 'colour', with a few Scots' words tucked in for 'authenticity'. It's an easy read, and flows along well, but it is also pretty superficial in its use of location. Eyemouth, if you accept the book's version is ringed by motorways (there are none); inhabited by ghosts (probably), and full of caring, soulful women and brawling, drunk fishermen practising 'free trade' (smuggling) - which is at best a part-truth.
I wouldn't have read the book if it hadn't been about the town and the area. Rosehill, the main setting for the archaeological 'dig' is based on Old Linthill. The likelihood of connections with the Roman Ninth are pretty dubious. My three stars is probably fair given that I'm not a fan of romance novels, primarily aimed at a female audience. For ghosts and ancient connections, Gordon Honeycombe and Alan Garner have written much more convincing novels, linking strands of the past and present.