As Good As God, As Clever As The Devil, By Rodney Bolt

Anthony Powell once came up with an ingenious theory of Victorian upward social mobility. Such ascents generally extended over three generations, he proposed. The first made the money and the second consolidated the social position. Come the third, decadence would set in, often accompanied by distinction in the arts, and an inability to produce heirs. Powell's template was the Firbanks: self-made railway-contractor grandfather; Tory MP son; and dandy-novelist homosexual grandson, Ronald.

With the family of Edward White Benson (1829-1896) this process was yet more concentrated. Benson was the son of a Midlands industrial chemist who died in 1843, leaving a widow, seven children, a patent for Cobalt and not much else. Iron-willed, intensely ambitious – and also temperamentally inflexible and buttoned-up – the teenage scholar embarked on a meteoric career that took him from the founding headmastership of Wellington College to the Archbishopric of Canterbury in 20 years. Queen Victoria doted on him ("the dear, kind, excellent archbishop" ran the telegram sent after his death) and commended him to her grandson, the future George V, as "such a friend of ours".

There were six Benson children, equally brilliant, volatile and neurotic. Martin and Nellie died young. Arthur (1862-1925) is remembered as a coruscating diarist and Elgar's collaborator in "Land of Hope and Glory". Fred (1867-1940) wrote successful society novels. Hugh (1871-1914) was a proselytising Roman Catholic priest.

Frequently their neuroses plunged into mania. Arthur spent long periods undergoing treatment for depression. Maggie, a pioneering Egyptologist, died insane. None of them ever married, or ever looked likely to, and their only descendants are a vast shelf of books. All this has been well-documented, in group and individual portraits, notably David Newsome's magisterial On the Edge of Paradise: AC Benson Diarist (1980). But no biographer prior to Rodney Bolt has ever got round to what, in some ways, is the most extraordinary story of all.

Mary ("Minnie") Sidgwick, the subject of Rodney Bolt's biography, was a mere 11 years old when her 23-year-old undergraduate cousin Edward Benson first took a shine to her. Within a year, despite a certain amount of maternal anxiety, there was a semi-official betrothal, whereupon Edward, whose punctiliousness in the matter of Minnie's intellectual development was painful to behold, set about moulding his intended's character.

They were married in 1859, the bride a tender 18. Looking back on her continental honeymoon from the vantage point of middle age, Mary noted: "Wedding night – Folkstone [sic] – crossing – oh my heart sank – I daren't let it... misery – knowing that I felt nothing of what people ought to feel... How I cried at Paris! Poor lonely child, having lived in the present only... The nights! I can't think how I lived."

Harrowing as all this is, it would be a mistake to mark Mrs Benson down as a sacrificial victim at the altar of the Victorian child bride. She admired and sympathised with her exacting husband ("He restrained his passionate nature for seven years, and then got me!"), pined to assist him with his great work but was frightened of his temper, as were the children. The hero of Arthur's first novel writes the words "I hate Papa" on a piece of paper and buries it in the garden. Later she embarked on a series of intense but essentially decorous female friendships, one of them with Dame Ethel Smyth, the advocate of Suffragism.

Bolt's technique in this panorama of Mary's life and times is to construct a running commentary made up not only of material from the Benson archive, Arthur's journals and Fred's autobiographical novels, but extracts from fiction and diaries. Mary's honeymoon, for example, is trailed by Amelia Sedley's wedding night from Vanity Fair and Dorothea Brooke's dusty dealings with Mr Casaubon.

Archbishop Benson dropped dead attending Gladstone's parish church at Hawarden. His relict survived a further 22 years, sustained by a relationship with Lucy Tait, herself an archbishop's daughter, who made a willing replacement in the marital bed. As Good a Good, as Clever as the Devil is an example of an accelerating trend in Victorian biography, the book that seeks to coax a hitherto neglected wife from the shadows cast by her domineering husband. But whereas Caroline Dickens – to make the most obvious comparison - is merely a representative mid-Victorian home-maker, Mary Benson is something else altogether: a woman whose personality shines off the page, and who seems at least as much an influence on her multi-talented children as croziered Cantuar. My only complaint about Rodney Bolt's consistently absorbing study is that it isn't twice as long.

DJ Taylor's new novel is 'Derby Day' (Chatto & Windus)