Kate Summerscale | "It threw up a lot of issues about the sexuality of women, and the admissibility of private thoughts and feelings."

Winner of the Samuel Johnson Prize in 2008, Kate Summerscale's The Suspicions of Mr Whicher was a runaway word-of-mouth success, with BookScan sales now in excess of 500,000 copies. Set once again in Victorian times, Summerscale's eagerly awaited follow-up Mrs Robinson's Disgrace (Bloomsbury, May) is not however another "whodunit?", rather, it raises the equally compelling question: "did she, or didn't she?" And instead of a youthful murder suspect, it's the heart and mind of a woman which are on trial.

It was while she was researching Mr Whicher that Summerscale came across an intriguing reference to a celebrated Victorian divorce case. "Although I couldn't do anything with it at the time, it stayed with me. It was a sort of puzzle. Once I had embarked on it, however, the book became not merely the solving of that puzzle but an attempt to reconstruct a life".

Mrs Robinson's Disgrace centres on the private diary of Isabella Robinson, a well-to-do but miserably married Victorian lady, whose face must "wear an outward calm though the fires of Etna boil within her breast". She proceeds to pour onto the blank pages of her journal her innermost hopes, fears and miseries; her intellectual hunger and sexual frustrations; her religious doubts; and her likings for other men.

Chief among these is Edward Lane, a handsome and brilliant doctor, and a married man. After several years of platonic friendship, but considerable lust and longing on Isabella's part, they have a short-lived and intermittent affair. Or do they? Therein lies a mystery that transfixed Victorian England.

Shortly afterwards, Isabella became ill. Her brutish husband took advantage of her indisposition to trawl through her things and discovered her diary, with its passionate account of her affair with Lane, as well as all her unconsummated yearnings. Mr Robinson subsequently sued for divorce, and the case—one of the first to be heard in the new Court of Divorce and Matrimonial Causes established in 1858—became a sensation, amid public disbelief that a respectable woman and mother would ever dream, not merely of committing adultery, but of committing the intimate details to a diary.

In the words of Summerscale, Isabella Robinson "wrote herself a drama in which she was the leading lady". The drawn-out nature of the case (it was not settled for nine months) is, she adds, evidence of how potentially threatening it was to the strict codes of Victorian society. "It threw up a lot of issues about the sexuality of women, and the admissibility of private thoughts and feelings."

The veracity of the diary was debated at lurid length both in court and in the newspapers, with Isabella's (and by association, Edward's) defence resting on proving that it was a fantasy; the work of a mad woman rendered insane by the effects of "uterine disease".

What gives the Robinson case a particular resonance is that it took place at a time of great uncertainty in Victorian society, with matters relating to marriage, sex, religion, and human consciousness all hotly debated in the intellectual circles in which Lane and Robinson moved. Summerscale fleshes out this fascinating backdrop beautifully; touching on the literature of the time (including Dickens, Wilkie Collins and Madame Bovary); the contemporary craze for diary writing; and the rise of such alternative practices as phrenology and homeopathy. Edward himself was a pioneer of hydrotherapy, and established a clinic reminiscent of today's Priory, specialising in the addictions and nervous afflictions of those who lived "hard and high".

So what does Summerscale believe is the truth of the Robinson case? "For a long time, I found the diary very ambiguous: a fascinating document but completely irresolvable. After coming across some other papers relating to the case, I became convinced that the diary was broadly true. But I hope I present the evidence in such a way that readers can make up their own minds. The fact that there is no absolute conclusion is the thing that gives it life".

This engrossing air of over-arching uncertainty is a trait that Mrs Robinson's Disgrace shares with The Suspicions of Mr Whicher. And Summerscale feels that there are other parallels, too. "Both books are close-ups on a forgotten and chronologically contained moment. I've tried to get under the skin of the characters again, and tease out all the connections between the social and artistic concerns of the time, and find in them the roots of our own preoccupations and confusions."

In place of a murder, the crime in Mrs Robinson's Disgrace is the vilification of an unhappy woman who dared to long for a different life. Reflecting on a diary passage in which Isabella Robinson begs an imagined reader to take pity on her plight, Summerscale concludes with the poignant thought that the readers of today might do exactly that.

"I felt that we could read her diary in a way that the Victorians could not have done. I wanted to be the reader she was reaching for".