Queen Elizabeth the Queen Mother, By William Shawcross

On 7 July 1959, Isaiah Berlin described a dinner party with Maria Callas and the Queen Mother. "I thought the QM not indeed particularly intelligent nor even terribly nice," he wrote, "but a very strong personality – much stronger than I thought her – and filled with the possibility of unexpected answers."

Berlin's appraisal, consigned to a footnote, is untypical of William Shawcross's official biography but overlaps to some extent with the Queen Mother's own self-assessment: "a very ordinary person" and "not as nice as I seem". Shawcross's focus is not so much the Queen Mother's niceness or otherwise – the verdict, repeatedly confirmed by primary sources, is one of potent charm – but her place in British life during her 78 years as a public figure.

Poet Laureate Ted Hughes examined in his "Masque for Three Voices", "how the Queen Mother [came] to be at the centre of Britain's experience of the drama by which the twentieth century will be remembered", namely the Second World War. Shawcross is concerned with the nature of her role centre-stage both before and after that conflagration. It would be unrealistic to expect startling conclusions.

Instead, this impressively researched biography demonstrates how the former Elizabeth Bowes Lyon won the public esteem which made her, alongside her husband George VI, a plausible wartime morale-booster, and how she retained the public affection won then through a further six decades.

Official royal biography involves pitfalls. Inherent in the diligent chronicling of daily engagements, foreign tours and posh frocks – "the Queen... wore a plain blue crêpe dress and cape and a spectacular hat with an ostrich feather plume, giving her extra height" – is the possibility of hagiography. Shawcross avoids the traps. Undeniably his biography has much of the encomium about it, but occasional flashes of mischief – he describes the Queen Mother's attitude to money as demonstrating "a certain insouciance" – help dispel the sugariness.

Elizabeth Angela Marguerite Bowes Lyon was born on 4 August 1900, the ninth of ten children of the future 14th Earl and Countess of Strathmore and Kinghorne. Although Elizabeth would later become England's first commoner Queen Consort since the 17th century, hers was an ancient family of unimpeachable aristocracy. The diarist Chips Channon referred to her as "so rare and patrician a creature". She counted among her forebears Robert II of Scotland and, more surprisingly, US president George Washington.

Her idyllic childhood was divided between Glamis Castle in Forfarshire, St Pauls Walden Bury in Hertfordshire and a rented Robert Adam mansion in London. It consisted, her brother David remembered, of "all the usual country-life things", but a limited amount of formal education. Lady Strathmore discouraged "advanced mathematics". Instead she sought to instil a deep Christian faith, a concept of British greatness her daughter would carry to her grave, and a series of self-help mottoes of the "life-is-for-living-and-working-at" variety. The young Elizabeth was in full sympathy with her mother, who would later tell her, "I think you know that you are far the most precious of all my children."

In this long book, it is the first quarter of the life which receives fullest attention. Shawcross's principal new material is the correspondence of Beryl Poignand, Elizabeth's governess from 1914 to 1917. The two women remained in touch until Beryl's death in 1965. By focusing on this period, Shawcross shows us the lively, vivacious and witty young woman with whom Prince Albert, Duke of York, second son of George V and Queen Mary, fell in love at first sight. "Bertie" proposed three times before receiving a favourable answer. What remains elusive here is the reason for Elizabeth's change of heart.

From the outset Bertie knew what Elizabeth could bring to his starchy and exacting family: "Life is not as easy... but the change is coming & you my little darling I hope are going to help me with this change. You must take them in hand and teach them how they should do these things." For her part, Elizabeth was under no illusions about the newly-renamed Windsors. "There is no 'family' feeling at all in this family. They are all very nice to me, & horrid to each other!" At home she set about charming her in-laws; on the public stage she had a larger but more nebulous task. She approached it with smiles and carried all before her.

In Auckland, New Zealand, a well-known Communist agitator foreswore his politics after a smile from the Duchess of York: "I've done with it for good and all." Through the next 70 years, by saying little and smiling much, this astute royal bride continued to win hearts and minds, testament to her instinctive "intelligence du coeur" identified by Sir Ronald Lindsay in 1937.

To the sceptic, smiling royal women are ten-a-penny. But this is an anachronistic dismissal. Before Elizabeth Bowes Lyon married the Duke of York, royal women seldom if ever smiled in public. The Tsarina Alexandra of Russia was known within her family as "Sunny"; her public face was anything but. Elizabeth's redoubtable mother-in-law Queen Mary dazzled with all the jewels of Empire: she saw no reason to light up a gathering by smiling.

Elizabeth's willingness to smile for ordinary people – and to do so both naturally and apparently sincerely – represents a minor revolution. It was a symbol of her ability to reach out and forge connections with those she had never met and would never know, part of a broader warmth which she deployed, for example, in the service of Britain's war effort. In Shawcross's reckoning, her radio broadcast to the women of America and conquest by charm of Franklin and Eleanor Roosevelt contributed to the States' eventual willingness to enter the war.

Like many biographers, Shawcross ends up treating his subject with a degree of leniency. He soft-peddles her animosity towards the Duchess of Windsor and excuses her lifelong resolve to avoid unpleasantness as proof of that determined optimism which contributed to her life-enhancing qualities. He succeeds in the difficult task of keeping his subject resolutely centre-stage in an elegant account which, though overlong, avoids being overwhelmed by those hoary old chestnuts, the Abdication Crisis, the war and the failed marriages of three of her six grandchildren. But is it disingenuous that the Duchess of Cornwall makes a single appearance – as the wife of Andrew Parker Bowles at a house party in 1973?

Matthew Dennison's 'The Last Princess' is published by Phoenix