NOTHING IN THE modern repertoire of entertainment has the mass appeal of an old-fashioned public execution. A hanging day, P.D. James and T. A. Critchley remind us, was "virtually a public holiday, and a vast crowd would congregate to see the condemned dispatched."

Few in the early 19th century "would have thought it justifiable to execute a man in private. The condemned had a right to a public death." Further, the hangman had a right to the body as well as to the clothes of the deceased, and he accepted bids for both, the churchyard often losing out to the dissecting table. The rope itself might fetch a shilling an inch from souvenir hunters.

Nineyine years ago a series of bloody, senseless killings of London streetwalkers, allegedly by a "Jack the Ripper," crowded out memories of the brutal Ratcliffe Highway murders of 1811. Unlike the Whitechapel butcheries of 1887, someone paid for the largely forgotten Thameside crimes. Still, those who knew any of the facts realized even then that the case had not been solved.

Already known for such novels as Cover Her Face, which became an instant classic of detective fiction, Phyllis Dorothy James, early in her career, collaborated with a police executive in the Home Office on her only nonfiction book. Thomas Alan Critchley was then a civil servant colleague. The Maul and the Pear Tree appeared in England in 1971, but James' American publishers remained uninterested, depriving readers here until this year. One wonders why. It is difficult to put down.

Investigation of a multiple murder in days before forensic medicine and scientific detective work, when even telltale footprints failed to be measured, and only enterprising newspapers collated all the testimony, was such an inadequately conducted operation that we may wonder how the authorities came so close. And as we wax impatient with them, we may sympathize with frightened informants caught in the web of suspicion, trapped by their contradictions, confused by the hodgepodge of urban jurisdictions in the days before the Metropolitan Police.

Following seven murders at the London Docks near Wapping Old Stairs, the appropriate suspects were interrogated, and some incarcerated. In a pit at the intersection of four roads near the scene of the crime, in the customary darkness, the traditional stake was driven through the hanged corpse of the culprit. His left leg was still weighted by prison irons. The grisly instrument that had thudded on the stake had been the bloodstained maul (or mallet) identified as one of the murder weapons.

In torchlight, jubilant thousands watched appreciatively as lime was sown, and the cobbles replaced. Justice appeared done.

For a week or two, residents of Shadwell and Wapping trod warily around the uneven paving stones, except children, who, "greatly daring, would leap upon the stones, then rush for cover, fearing equally their mothers' wrath and the contamination of those awful bones."

AS WITH MOST tales of detection, the unraveling of the evidence is more dramatic than the carrying out of the crimes. The young linen draper and his family, the middleaged publican and his family, were only victims of the society in which they carried on their business. The Thames wharves and warehouses, seen from a distance in the early prints of Whistler, possessed a mysterious, raffish attraction. In truth, Dockland was a grim, lawless society, lorded over by a "swaggering, disreputable aristocracy" of seamen. East London lived on what sailors brought, and what sailors bought. Beggars, thieves and legitimate businessmen survived precariously in shops and shacks and teeming lodging houses, little more than hovels, like the Pear Tree. It is a venue described by James and Critchley with a sensory immediacy that manages to eschew melodrama. You are there. Some of the story has been told before, although never with the richness of detail that gives The Maul and the Pear Tree its period texture. The opium-eating Thomas De Quincey offered his fanciful version in a postscript to his On Murder Considered As One of the Fine Arts (1854). But the authors quote an early opinion on De Quincey that he was "happily immune from a pettifogging intimacy with dates, names, and trifling matters of fact." Their method is far different.

Perhaps the most striking aspect of their strategy is an insistence upon questioning every particular of motive or circumstance. What makes an unbloodied crowbar evidence? ("It was almost certainly brought into the house by one of the murderers. . . .") Why does a neighbor ignore cries at midnight? (The murdered young couple were married only a year.) Why is a suspect kept in his cell when cleared of guilt? ("Having brought him at public expense all the way from Godalming, it would be extravagantly generous to let him go after so little incovenience.") Why trust a child's testimony? ("Children . . . have a keen eye, a retentive memory for anything which interests them; and, being untroubled by the irrational doubts and divided loyalties which can afflict their elders, usually give their evidence simply and without self-consciousness.") Was a crucial knife used in both sets of killings? ("A bloodstained knife left uncleaned for twelve days would hardly have sliced so efficiently through . . . stout throats.")

It seems impossible to separate elements of the collaboration, although one may suspect the wry wit as that of P.D. James. What matters is that the book evokes more than a butchery; it brings to vivid life the teeming netherworld of Dockland London at a time when theft of s5 or its equivalent in goods was as much a hanging crime as a murder, when twelve shillings a week (then about three dollars) was a good workman's wage, when the appearance of a "chemical gentleman" to validate a bloodstain was a rare and novel element in the annals of crime. With relentless understatement, The Maul and the Pear Tree moves as close to a solution of the 1811 crime as we are likely to have, and in the process recaptures a time and a place since erased by firebombs and other agents of progress.