The chief exhibits from Kate Summerscale\'s deservedly bestselling and prizewinning last book, The Suspicions of Mr Whicher, are still hanging about rather grotesquely in my mind. The mutilated body of a boy in the water closet, a sash window slightly open, a bloodstained nightdress stuffed into the boiler. Summerscale\'s reconstruction of the infamous murder investigation at Road Hill House was also a cross-examination of Victorian domestic life and its most disturbing secrets. Now, in Mrs Robinson\'s Disgrace, she uses the same techniques of historical sleuthing to reconstruct the events that brought one couple to the divorce courts in 1858. As before, she follows the clues outwards from this one case to the larger anxieties, prejudices and cover-ups that shaped it. Summerscale puts Victorian middle-class society back in the dock, and again it is both horrifying and salutary to follow the questioning from the gallery. The chief exhibit this time is a diary. No blood, no corpse in the privy, just an ordinary Letts diary of the sort that is currently half price in Smith\'s. But this too is a relic of passions that could not be contained and which are exposed in the end to the scrutiny of a nation. The unfortunate diarist, Isabella Robinson, fell in love with a man who was not her husband and wrote her feelings down. That was her crime, that was her ruin, and that was all it took to cause a scandal of major proportions. Her husband, the industrialist Henry Robinson, had a range of mistresses and two illegitimate children, but no matter. For a woman the standards were different. After all, she didn\'t even own her diary. The man she loved was the married doctor Edward Lane, pioneer homeopath and proprietor of an advanced hydropathy establishment in Surrey where patients were prescribed a bewildering number of different kinds of bath. Isabella spent time at Edward\'s spa, and in his study, but did they embark on an affair? Nothing in Edward\'s letters proved it – he was too careful a correspondent. Isabella, on the other hand, wrote in her diary a day-by-day narrative of her erotic longing and the dreamed-of reciprocation that began one afternoon in the Surrey countryside when Edward turned to her on the plaid picnic rug and kissed her. In her diary Isabella was the heroine of her own romantic novel. She described the misery of her marriage to the insensitive Henry, her ennui and entrapment, the great redeeming joy of evenings spent reading poetry with Edward. She wrote out her fantasies and expressed the full force of her desire. She stopped just short of recording everything (\"I rested among the dry fern. I shall not state what followed\"), but she included in her diary much more than was licensed in any contemporary fiction. In France, Gustave Flaubert was at work on his great novel of adultery, Madame Bovary, but it would be banned from publication in England, deemed too repulsive and corrupting for English eyes. Henry Robinson was one of the first to sue for divorce under the new Matrimonial Causes Act of 1857, a landmark act that for the first time made divorce a possibility for middle-class couples. If Mr Whicher\'s investigation at Road Hill was the original whodunnit, Robinson v Robinson & Lane was the \"original\" divorce, the ancestor of more than 100,000 divorces that took place in the UK in 2010 alone. And what an agonising mess it was. Isabella\'s diary was proof of her lustfulness but it did not prove anything conclusively against Edward. Could it be used against her and not against him? The only way to save Edward\'s reputation (as everyone was eager to do, at Isabella\'s expense) was to discredit the diary as the raving of a sex-obsessed lunatic. By a skewed logic more perverse than anything Isabella could dream up, a sane woman was now reinvented as an erotomaniac driven mad by a conveniently identified uterine disease. At every stage this \"real-life\" story is a skein of fictions. Coleridge and Shelley taught Isabella what a love affair might be, and how she might construct herself as its heroine. The lawyers, too, learned their lines from literature. In court they fantasised a gothic tale in which Isabella\'s madness poisoned a whole respectable milieu. Summerscale is a subtle interpreter of the interplay between action and literary imagination, as was clear in Mr Whicher. A large part of her fascination with the Road Hill case lay in its influence on subsequent detective fiction. If the literary connections in Mrs Robinson are less compelling (and it should be said that Isabella is not going to vie with Emma Bovary for literary immortality), they are crucial to the vivid anatomy of an 1850s mind. Summerscale might have been more upfront about her own methods of narration in a book so concerned with reading, writing and the interpretation of documents. There are potentially significant gaps and doubts in a story she strives hard to render as a polished whole. It comes as a surprise, for example, late in the day that Summerscale has not read Isabella\'s diary, the fateful book having been lost or, more likely, destroyed. She has had to piece together her account from extracts published in legal reports. She has made careful use of correspondence, but how representative are the letters that survive? Foraging in the footnotes in an effort to find out, I couldn\'t help feeling that discussion of these matters would have left us better equipped to read Isabella and her times. As a guide to mid-Victorian cultural life, however, Summerscale is simply superb, and she sets a fine example of what cultural history can do. Isabella has her head examined by a phrenologist, so we get a miniature history of phrenology and its implications. (A large cerebellum meant excess \"amativeness\"; Isabella\'s cerebellum was very large indeed.) To understand Edward and his Moor Park spa we need to know about hydropathy, so we go on a course in alternative medicine and curative bathing (chair baths, hot air baths, wet towel baths, secretly sensuous baths). In other hands, admittedly, this might become tiresome. But Summerscale has a knack of judging just how much we want to know. So she keeps adding strands to the web: divorce law, diary-writing, Victorian dream theory, gentlemen\'s advice on the advantages of erotic encounters in bumpy carriages. She knows that the settings, too, are eloquent: Henry\'s big white villa in Caversham where nobody is happy, the sandy soil of the Surrey hills, the precise qualities of Edward\'s study with its many doors, and the foul smells from the Thames that filter into a hot Westminster courtroom at the stinking centre of the British empire. Sensing a silence or slight misunderstanding between two characters, Summerscale prods a bit, and the door flies open to a whole new set of stories. It\'s like watching someone going straight to the secret compartment in a many-drawered cabinet. Edward\'s friend and brother-in-law, George Drysdale, needn\'t have figured at all in Isabella\'s story, except that he turns out to cast a shadow across the whole affair. He faked his own death out of shame for his sexual fixations, resurrected himself, failed to cauterise his penis, and went on to write the first guide to contraception. That\'s the kind of obscure but astonishing life story we keep glimpsing in the background. And we glimpse too, at a distance, famous people going about their business, like Hamlet in Rosencrantz and Guildenstern Are Dead. When not decoding Isabella, the phrenologist is feeling George Eliot\'s cerebellum; Charles Dickens is leaving Boulogne just as Isabella gets there; and who should be strolling at Moor Park but Charles Darwin, so relaxed by his bath treatments that he said he \"did not care one penny how any of the birds and beasts had been formed\". He didn\'t stop caring for long, of course: he was at work that year on The Origin of Species, refining his theory of evolution even as Isabella reconciled herself to atheism and wondered what the absence of God might mean for the future of sexual relations. At every turn Isabella\'s experience is contiguous with that of the people who were deciphering and shaping her world. But ultimately it is Isabella herself who stands as exhibit A in this engrossing investigation of a society casting judgment on itself and trying, with much confusion, to make up the rules.