Married as I am to an antiquarian book dealer, and living in a house infested with books and manuscripts, I’m constantly having to edit my own little library so as to be able to breathe. But three volumes have survived successive culls — Pax Britannica, Heaven’s Command and Farewell the Trumpets — Jan (or James as she was when these books were written) Morris’s trilogy about the British empire. It is, Morris says, ‘the intellectual and artistic centrepiece of my life’, and it opens on the morning of 22 June 1897 with Queen Victoria visiting the telegraph room at Buckingham Palace on the occasion of her Diamond Jubilee.

She was, Morris tells us:

wearing a dress of black moiré with panels of pigeon grey, embroidered all over with silver roses, shamrocks and thistles. It was a few minutes after 11 o’clock. She pressed an electric button; an impulse was transmitted to the Central Telegraph Office in St Martin’s le Grand; in a matter of seconds her Jubilee message was on its way to every corner of her Empire.

From these few sentences alone, you can begin to see why Rebecca West thought Morris ‘the greatest descriptive writer of her time’. She draws us immediately and irresistibly into the story, and while it’s clear that her canvas will eventually reach as far and wide as the Queen’s Jubilee message, she keeps us anchored and alert through precise details: the panels of pigeon grey, the exact time.

Derek Johns was Jan Morris’s agent for several decades, and, while not authorised, this ‘literary life’ has been written with her blessing. In chapters that are thematic rather than strictly chronological Johns quotes liberally from Morris’s 40-plus books — or what he reckons are three to four million published words — and from her journalism (which kicked off in 1950 with a piece for The Spectator). He has a gift for selecting texts that drive you back to read, or reread, the originals. As early as page ten, in an opening chapter on Oxford, he quotes a passage on the city observed from the Fellows’ Garden in Exeter College at dusk on a winter’s day. It combines, as only Morris can, nostalgia and elegiac glee, and makes one confident, with Johns, that Oxford is ‘surely the best book ever written on the subject’.

But what Johns offers is much more than a patchwork of extracts. His own prose is elegant and his insights incisive, and though he is affectionate he is not always uncritical. Jan Morris can baffle readers with an overabundance of facts; she has a tendency to pass over unpleasantness; and in retrospect her political views can make you wince. On a visit to Johannesburg in 1957 she asks awkward questions about the wisdom of ending apartheid.

‘I spent half my time travelling in foreign places,’ Morris writes in Conundrum (1974), but Johns gives the impression that it was much more than half — that, ever since her mischievous and brilliant reporting of the Everest expedition in 1952, Morris has scarcely stood still. There seems almost no corner of the earth she hasn’t written about, and South America is the only place she’s failed to warm to. So exhaustively travelled is she that she has had to invent a country, Hav, to delight and bewilder her readers.

Behind this compulsive wandering there is perhaps something more than wanderlust. Aged three or four, sitting under the piano while her mother played Sibelius, James Morris decided he was really a girl. The conviction never left him, but he was well into middle age — a husband and father of four — before he embarked on hormone treatment, preparatory to full surgical gender reassignment in 1972. Jan Morris emerged from this with ‘a marvellous sense of calm’, but along the way, Johns reckons, there had been ‘clear signs of torment — torment sublimated in different ways but never entirely resolved’. If it was this, in part, that made it so difficult for Morris to stay put, we should be selfishly grateful. It has brought us some of the best British writing since the end of the second world war.

In person, Johns says, Jan Morris is much less extrovert than in prose. Now 90, she would probably hate the idea of a full-blown biographer picking over her life. But what Derek Johns has achieved is as satisfying as any biography. Slim, sympathetic, and beautifully illustrated with Jan Morris’s own line drawings, Ariel has joined the empire trilogy on my shelves, and won’t be turfed out.