AT the beginning of her 10th decade, Jan Morris decides to “have a go” at keeping a diary of her thoughts. “Good luck to me,” she declares, before rattling off at the same sprightly pace with which she completes her daily walk: 1000 paces each morning, orchestrated to a stirring mental, or occasionally whistled, rendition of La Marseillaise, I’m A Yankee Doodle Dandy or some other “brisk piece of nationalist blarney”.

In committing her ruminations to 320 entertaining pages, Morris creates a captivating image of a delightfully dotty elderly lady, whose day begins with a conversation with her toothbrush and ends with a thank-you to her furniture and an agnostic but sympathetic message to “whatever almighty power” may be up there trying to sort out our messed up world. “Goodnight, God,” she says, “and good luck to you!”

A former soldier, journalist and author of more than 40 books, Morris has led an adventurous but highly disciplined life and appears to be revelling in the licence to eccentricity afforded by old age. Then again, this 91-year-old wordsmith remains powerfully in control of her prose and her playful self-portrait of the artist as a potty old bat should not be taken as read. This writer is as sharp as they come.

In My Mind’s Eye reads as a series of pithy little essays, each exploring a thought that occurs to the author in the course of a daily life spent in the corner of North Wales she’s inhabited for most of her life. Here, for instance, is her conscience arguing with “my personal Beelzebub” as to whether she should bother rescuing her absent neighbour’s laundry from the washing line during a rainstorm. (Her neighbour, Beelzebub reminds her, is a “goody-goody” who thinks she’s better than everyone else, and the washing goes un-salvaged.)

And here she is sitting in a local cafe earwigging on two posh people’s conversation about shortbread, pondering the evolution of received pronunciation and wondering if she, too once spoke with the “ineffably affected” accent of the young Queen Elizabeth, who was born in the same year as Morris, 1926.

All very parochial, you may think, especially given Morris’s record as an intrepid foreign correspondent who famously reported live on the conquest of Mount Everest, filed world-altering dispatches from Cyprus on the Suez Crisis and produced celebrated books on Venice, Trieste and the US.

Yet if her sphere may seem diminished, her focus is as broad as ever as she cogitates on the direction of travel of a world ruled by Trump, Brexiteers and burgeoning artificial intelligence, albeit from the viewpoint of a radio listener rather than a roving reporter. And after all, that perspective has been sharpened by a life spent exploring barely charted territory.

In My Mind’s Eye makes no mention of it but in fact, the author spent the first half of her life as James Morris. “I was three or perhaps four years old when I realised that I had been born into the wrong body, and should really be a girl,” she famously wrote in her 1974 biography Conundrum. And although James forged a successful career and enjoyed a happy marriage to Elizabeth Tuckniss (with whom he had five children – one of whom died in infancy), in 1972 he became Jan after undergoing one of the world’s first sex reassignment operations. Having had to divorce before the surgery (since same-sex marriages weren’t then allowed), she and Elizabeth carried on living happily together until, in 2008, they re-cemented their union through a civil partnership.

And blissfully happy they appear to have remained. “Only now,” writes Morris in her thought diary, “is that subtle demon of our time, Dementia, coming between us.”

Elizabeth’s conversation, we learn, is narrowing. “She forgets a lot, and crossly denies that she has forgotten it. She can still summon her old charm for outsiders, but saves her irritations for me.” Dementia, she adds ruefully, “brings out the worst in me too. It is a two-way evil, it incites me to harshness and impatience, and to say unpleasant things I really do not mean”.

Here, I think, is where we catch an authentic glimpse of the tough challenges confronting the author – and indeed, all of us, as we approach our advancing years.

For while her book reads largely as a joyful romp through old age, taking sprightly swipes at neighbours, world leaders and sacred cows along the way, Morris allows herself the occasional despairing aside about the “miseries of nonagenarianism”, bemoaning aches, pains and fear of her own encroaching senility, which she detects in her increasing authorial dependence upon the exclamation mark.

“Poor me,” she laments, after opting out of a broadcasting commission on the grounds of “old age, shaky health, fading powers and personal anxieties”. But then: “Do shut up,” she chastises herself, before anyone else can accuse her of self-pity – and in Morris’s deft hands, none of this is gloomy and her spirit shines brightly.

When someone asks her to share her recipe for a happy old age, Morris reflects on the rashness of the assumption that she is enjoying one, yet gives an answer that’s far from curmudgeonly.

“Be kind,” she says, echoing an exhortation she makes throughout her book, and that’s clearly the principle by which she tries to live even if, as demonstrated by Beelzebub’s victory in the washing line dilemma, she doesn’t always entirely succeed.

Thank goodness, though, for her barbed keyboard and wicked sense of humour, which kept this reader – a fellow sufferer of what Morris describes as the “affliction of tunes in one’s head” and a committed member of the “besmirched minority” of inveterate whistlers – nodding and chuckling along to the last page.

After closing the book, I felt bereft of Morris’s company and her chummy, conversational prose style. Don’t you agree? she occasionally asks the reader – and at times, you find yourself wanting to say, “No, actually ...”, particularly when it comes to this committed Welsh republican’s baffling nostalgia for Britain’s long-lost “greatness”, or her fondness for “the swank and glory of the old British Empire”, whose history she charted in her 1968-1978 Pax Britannica Trilogy.

Then again, you can’t help enjoying the way that she says all this, and Morris herself appears well aware of the contrary nature of her affection, admitting that the empire “was founded upon fundamental injustice” and that capitalism – whose “biscuit makers, chocolates and paternal country bank managers” she used to admire - is after all “just one vulgar scam”.

In a recent interview, Morris said that her last book will be a posthumously published collection of allegories, but it seems to me that her allegorical powers are already on display in this wonderfully wise collection of musings.

What else are we to make of this far-from-batty 91-year-old’s description of chasing the rainbows which, she writes, have recently been appearing in her neighbourhood, prompting her to jump into her car and seek out “the patch of glimmering glory where the rainbow ends and the pot of gold lies … just at the end of the lane, just over the river, just behind the Parrys’ farmyard – until, ah, just before I reach it, it is never there after all”.

Then she listens to Judy Garland singing Over The Rainbow, and tears come to her eyes.