I wonder if the blurb on my advance copy doesn’t undersell this novel and its predecessors.

"Cosy crime with more than a dash of humour, Catriona McPherson’s Dandy Gilver series is the perfect next step for fans looking for a successor to Dorothy L Sayers and Agatha Christie."

A blue-blooded sleuth certainly (a woman this time, not a man), and set in the Golden Age of British crime fiction. No psychopathic killers or slaughterhouse gore, and yes, a prevailing wry wit.

But the books offer a literary grace and poetic turn not normally associated with crime titles. Each fixes on some particular social milieu and colours it in. The psychology is sharp; physical descriptions are spot-on. I would argue that it’s their (a big word) humanity which also enables them to transcend genre.

I am a recent convert, and making up fast for lost time. TV has optioned the books. Here is the new ‘Dandy, the twelfth title, and as in the last seven the name of our intrepid narrator comes first.

The promotional material’s resume can’t be bettered, so I’ll cheat and let it do my work for me.

"Fair is foul and foul is fair as aristocratic private detective Dandy Gilver heads off to Castle Bewer to solve the mystery of a missing ruby necklace and a tragic family curse.

She arrives as the residents are preparing to stage a production of Macbeth, yet sinister goings on seem to be more than amateur dramatics."

The Scots author is now based in northern California. If anything, this move seems to have sharpened her focus on her homeland.

I enjoy her natural descriptions especially. She has retained an eye for landscape and weather, and how can a part time-countrywoman like Dandy Gilver not be observant?

"It was one of those dour days in a Scottish June when the sky rumbles purply and the rain batters flat the straggling meadowsweet in the hedgerows. Only the midges are happy on such a day and we slapped at them tirelessly as we trundled along."

There is a distinct sense of period, and while we learn for instance that Dandy drives a Morris Cowley, and this and that, the atmosphere isn’t of the kind heavy on nostalgic Ovaltine-y brand-naming. The McPherson ‘mood’ has to do with customs and rituals, and with the way people address each other, and with a more formalised – or merely more precise – mode of speech. The characters feel alive, thought-through, and aren’t there (as so often in fiction) as totems and symbols or as mannequins in retro-fancy dress.

Nanny Palmer for one, in her Bewer estate cottage, walks in from the tram-lines, so to speak, and a few sentences capture her:

"She was an ancient woman, or at least appeared so. Bent like a crochet hook and with a face lined in stripes like mattress ticking from brow to collar."

And that row of cottages is seductively described, somehow filling the mind:

"The front gardens were a picture … the same effusion of roses was rioting in every little patch, with the same carpet of columbine below, the same musky currant bush at each gate and the same lavender lining each path. It was delightful to the eye, if rather overwhelming to the nose …"

It’s easy for modern writers to be snide about the past, or patronising, that is when they aren’t seeing it as heritage-quaint; it takes much more imagination to inhabit the period. (Why does Catriona McPherson’s Scotland feel more Scottish than the country does to me now?)

The books have progressed through the 1920s and 1930s, year by year, past the Armistice and General Strike and Depression and other defining national events. (In the latest, it’s been a total of twelve years since the setting up of the Gilver & Osborne practice by Dandy and colleague Alec.) 'Returning’ detectives in fiction frequently tread much the same water as before, they alter little if at all from one book to the next. The advantage of forward movement in this series of novels is that it allows our heroine to mature. Mrs Gilver is now "over forty, a matron with grown sons"; we gain welcome new insights into her life at Gilverton with younger husband Hugh, sharing their home with long-serving staff like butler Pallister and personal maid Grant and memories of family notables and beloved pets.

It’s unfashionably civilised fiction. The plotting has become very slick and confident, and here subtly complements the developments in the drama that is being performed, Macbeth. Lines from the text are quoted, unapologetically, as if we readers are up to snuff on matters literary, thank you. The introduction of some wealthy American ladies-of-a-certain-age is further counterpointing, allowing more of the author’s always warm humour just as Shakespeare’s play heads for its moral nadir.

Catriona McPherson is charitable to all her characters, including even the ones who spell danger. I was set thinking of French film director Jean Renoir; he reminded us in his 1939 film, The Rules of the Game, about some ultimately very dark theatrics at a chateau house-party, ‘ … everybody has his [or her] reasons’. (Meaning, I take it, that criminal deeds will often stem from a sincere, if misguided, sense of our personal destiny.)

In the true tradition of Agatha Christie the solution to the inconvenient little problem of murder – murders plural – lies somewhere among the offshoots of a family tree, in that family’s tangled past associations with a second family. The Bewers have long lived in moated splendour, which fact – of having a castle moat to their name, rather than the splendour aspect – has a bearing.

Should you act on my wholehearted recommendation (no quibbles), you will have the pleasure of discovering so for yourself.