In the 2013 Time magazine profile that grew into her book about Prince Charles, Catherine Mayer assured readers, “Much of what you think you know about the prince is wrong.” Perhaps. Her greatly expanded portrait suggests that a whole lot more of what we think – or fear – we know about our future king is entirely correct, not invariably in a bad way.

He works hard. Although often at the strangest or most undesirable things. He wants to make a difference – even in places where he has no business interfering. His philanthropy is impressive, albeit so chaotic as to defy meaningful summary (Clarence House has lost count of his charities). He has a strong sense of duty – and zero awareness of his political illegitimacy. His thoughtfulness is only matched by his gullibility. He wants to change the world – right up to the boundary of the tax-immune Duchy of Cornwall. His judgment of character has allowed friendships to flourish with, among others, Sir Laurens van der Post, the creepy fantasist, the paedophile Jimmy Savile (who called him “his nibs”) and the sharia-loving Saud family. Even Charles’s friends call him “Sir”. He hankers for the antique social structures of deepest Transylvania. Some of his enthusiasms make the late Madame Blavatsky sound sane and rational. His worries about sustainability, and ostentatious frugality, are no disincentive to insatiable house-acquisition, extravagant staffing and pointless air-miles. He is doted on by people as clever as Emma Thompson. And yet, displaying the kind of unchanging nature that is so admired in his mother, still Charles whinges for England.

“Each thing I did, you had to meet another lot of people who have all sorts of views of you beforehand, all sorts of prejudices,” he wittered at a broadly sympathetic Mayer in their original interview, which he declined to supplement with a second.

If Mayer does not, as promised, “puncture those prejudices in order to close in on the far more interesting truth”, maybe this is because they were not, in fact, prejudices about Charles, so much as very reasonable suspicions, some of them based on the Jonathan Dimbleby biography and programme of 1994, some on his subsequent speeches, letters and conduct. The most pressing questions about Prince Charles, for loyalists and republicans alike, relate to his capacity to adapt, to a changing nation and a constrained job. Can he ever, like his mother, embrace his role as a figurehead, or will the Caroline perception of kingship require us all, come Christmas, to endure vatic exhortations on the themes of nature and golden threads, the “beingness of things” and another fad, “perennialism”?

Denied his answers, Mayer, having performed the requisite acts of submission, including the curtseys this great visionary hilariously expects from fellow adults, does amass some cherishable revelations. Who knew, for example, that the prince is revered for his peerless bladder control? “He knows exactly how to hydrate his body to just the right degree,” his godson, Nicholas Knatchbull, volunteers, perhaps unaware that the once sought-after position of Groom of the Stool was discontinued in 1901. “It’s an incredible talent.”

Ditto the prince’s dancing: “I’ve never danced with anyone who can actually lead me, and I can just relax and go, this is great, this is better than sex,” Emma Thompson told Mayer, in the course of a eulogy that seems only to have omitted her old friend’s mastery of breathing and sleeping. “He’s a very charismatic, virile dancer.”

Added to extensive evidence of potentially republic-hastening idiocy that Mayer – a mildish Charles supporter – believes are outweighed by his virtues, her accumulation of authorised tributes, each one more gushing than the last, until they explode in a final, doubt-deafening chorus of servility, is one of the most compelling reasons to reject any assurances that, on inheriting the throne, Charles will be able to stop himself from trashing it. It evidently remains a condition of successful courtiership never to stop hailing “sir” as a synthesis of visionary, saviour and gifted dance artiste, sent to lead his people out of science-oppressed darkness.

Admirers and intimates (half of whom wanted to run their praise past “HRH”) testify that sir throws “the best parties”, sir is kind – no, actually sir is Christ-like, “almost to the point of pain he suffers with people”, sir is emotionally intelligent, a “huge asset” to the FCO, sir is “trying to save the world, dammit!”, sir is like Richard Branson, also Steve Jobs, although “almost too nice”, a man of conviction, with “a wisdom about him”, venerated in Armenia, a “fantastic bloke”, ahead of his time, and, to one particularly besotted acolyte, “a remarkable human being” whose arrival “on the planet” at this moment is “no accident”. And supposing he’d arrived too early or late to redeem mankind, Mayer herself tells us that with his great comic timing, sir could have been a decent standup.

Even when Charles is, by any standards, having a laugh, probably at the expense of some hapless scientist or architect, Mayer invites us, as Dimbleby did, to make allowances for the royal survivor of frigid parenting and school bullying, doomed to languish – assuming he couldn’t just quit – within the “prison shades” of sovereignty. If superficially, Britain’s premier victim, “heir of sorrows” as Private Eye used to call him, looks quite well set up in Highgrove and his seven or so other properties, with his old girlfriend primed for queendom, that’s only because “he has accepted”, Mayer points out, “a life of permanent homesickness, shuttling between houses which he inhabits but apart from two cottages in Romania doesn’t strictly speaking own”.

And by way of a somewhat ungallant coup, she discloses that Diana was not alone in pre-wedding jitters: the equally damaged Charles, according to his friend Nicholas Soames, was no less a lamb, or, at 32, mutton, to the slaughter, “because she started with the bulimia and everything before the wedding”. It’s as if almost no time had passed since the night of the Panorama interview, 20 years ago, when Soames diagnosed in the princess “the advanced stages of paranoia”.

If, as is to be hoped, the Guardian is successful in exhuming his letters to ministers, we may shortly enjoy some further, not necessarily encouraging, insights into Charles’s interior life. It is regrettable, though all too characteristic, that neither Charles nor his advisers recognised the arrival on his planet of a journalist as shrewd, respected and notably well-disposed as Mayer, as a divinely ordained opportunity for him to say, in somewhat more detail than Shakespeare’s Henry V, “Presume not that I am the thing I was.”

Charles: The Heart of a King is published by WH Allen, £20. Click here to buy it for £16