4 stars for an entertaining historical fiction book.
The premise of this book is that, at the start of the German bombing blitz of England, Princesses Elizabeth and Margaret are whisked off to Ireland for their safety. Actually according to her Wikipedia biography, they spent this time at Windsor Castle. Windsor Castle is a formidable Castle. I took a visitor tour in 2016.
In any case there are richly drawn characters:
Strafford, the only Protestant Garda detective in the Irish Garda(police) in 1940.
Celia Nashe, one of a few females accepted into the Secret Service
Thomas Clancy, hardware store owner in Clonmillis and IRA wannabe
Lascelles, an arrogant Diplomat from the Dublin British Embassy
Sir William Ormonde, the Duke of Edenmore, owner of Clonmillis Hall, where the princesses stay.
These characters and others interact in a story more of a historical fiction/character study than a mystery, since you know that the princesses survive.
I enjoy the author's evocative use of language to describe both the characters and the settings.
Some quotes:
Arrogance: "Lascelles had noticed that the detective had hardly bothered to look at the house, probably knew a dozen places like it--Strafford was a bona fide descendant of sixteenth century Protestant settlers. But if so, what the hell was he doing in Paddyland's pretend police force, which was hardly more than a gang of pensioned-off gunmen pressed into blue serge uniforms and told to behave themselves?"

Conversation between Nashe and Strafford:
"Yes, I mustn't let my imagination run away with me,' she said. "Who would want to harm two young girls?"
To this, again, he made no reply; if he were to respond with what he really thought, it would be no comfort to her. Ireland had been held in British overlordship for eight hundred years, more or less, depending on who was doing the counting, and although the larger part of the country was independent now,the fact that it had been occupied for so long had a potent, abiding and visceral significance for a considerable portion of the population."

Clonmillis Hall: "For years this house had drifted along contentedly enough, like a giant anchorless hulk in a torpid sea."
Thank You Henry Holt and Co. for sending this eARC through NetGalley.
#TheSecretGuests #NetGalley

Update November 24,2019. My wife agrees with me on the 4 stars, but found the author was too verbose. She said that he used obscure words to impress the reader, i.e. comestibles, instead of food.
We both believe that one should write to express not impress.