BOOK REVIEW : From London’s Seamier Side, a Tale of Woe, Betrayal : THE VAST MEMORY OF LOVE, <i> by Malcolm Bosse,</i> Ticknor & Fields, $21.95; 482 pages

“After London, I’m not the man I was”--so says a young thief in this historical novel when brought before Henry Fielding, writer and city magistrate. The words strike a chord in the reader because they echo those of Ned Carleton, the hero of “The Vast Memory of Love,” who in the prologue to this book wishes he could say to his dead father, “I never lived till I saw London.”

Like the thief in Fielding’s court, Ned is a former shepherd turned to crime because he sees no other way of surviving in the city, but unlike his countryman, will soon experience the absolute worst that London has to offer. Betrayed by his new city companions, Ned ends up in the stone hold of Newgate Prison, a convicted murderer awaiting execution.

Malcolm Bosse, who has made a career of writing long, densely populated novels such as “Mister Touch” and “The Warlord,” here takes on 18th-Century urban England while simultaneously paying homage to the English novel itself--which magistrate Fielding, as the author of “Joseph Andrews” and “Tom Jones,” helped create.

Bosse gives Fielding a fateful role in “The Vast Memory of Love,” and although the novel lacks the humor found in Fielding’s work--there are smiles, but no laughs, in its nearly 500 pages--Bosse makes up for such austerity with bloodcurdling portraits of the Earl of Sandwich and Robert Scarrat, the men who set Ned on the road to crime.

Sandwich first appears as a benefactor, having employed broad-shouldered Ned as a coachman only minutes after the shepherd arrived in London, seeking his fortune, from his native Nottingham. Ned flourishes in the job, but loses it when he refuses, being innocent, to admit to charges that he has stolen from the kitchen larder. Cast out, Ned happens upon a house afire and, in his failed attempt to rescue a woman trapped inside, burns his left hand almost beyond use.

When Scarrat, Sandwich’s spy and procurer, recognizes Ned, he merely mocks him, saying “You can’t last long at a simpleton’s game here. This be London, boy. How do you like it?” Scarrat shows sympathy unexpected even to himself, however, when he hands Ned a half-burned copy of “Joseph Andrews” found lying in the street.

Only gradually does Ned realize that Scarrat’s advice is sound and never that Scarrat’s strange gift may save his life. Unable to find employment because of his damaged hand, in desperation he turns to petty crime, stealing food and then scarves.

Ned remains a minor thief until he befriends a wild, fierce collie and trains it as he would a working sheepdog. The training begins as a sentimental act, but Ned soon puts the collie to work by having it corral unsuspecting victims into dark alleys--and thus transforming himself into the notorious Dog Cull, able to rob three, even four men simultaneously without showing a weapon.

Ned isn’t proud of his new skill and fame, however, and when he falls in love with Clare, a secretive seamstress, he determines to commit one last crime that will allow them to leave London and start anew. His chosen victim? Sandwich, about whom Ned has gleaned some damning information.

Ned’s scheme, of course, does not proceed according to plan, with any number of acquaintances willing to forsake him for spite or gold. Ned does get money from Sandwich, but in the course of the transaction a man dies, and Ned is arrested as a thief and murderer. His conviction quickly follows, for Ned has no defense to make: One word about Sandwich, Scarrat has told him on a prison visit, and Clare’s life is forfeited.

Like Scarrat, however, Fielding, too, is a frequent visitor to Newgate, and he comes to believe Ned’s story has not been fully told--a belief bolstered by the fact that Grub Street’s notorious Dog Cull can show the novelist-magistrate his cherished copy of “Joseph Andrews.” With the clock ticking on Ned’s life, Fielding pursues Clare, amorously, and Scarrat, legally, and you can guess the rest.

Or much of it, at least, for although “The Vast Memory of Love” unfolds like an 18th-Century novel, its particulars are harder-edged and more disturbing. Ned’s London is “a world of ruins, tilted hovel and crumbling shanty, an undersea graveyard of shipwrecks rising from the mists like the hulks of yesteryear”; Newgate a place where women prisoners are advised to submit to rape in order to “plead your belly at court”; Scarrat a man who understands “Hatred was a close ally of cunning.”

Not everything works in “The Vast Memory of Love”--some of Ned’s story is told through Fielding’s diary entries, and the device seems both unnecessary and intrusive--but on the whole the novel is an admirable achievement. Bosse appreciates that mean-street life has its counterpart in the salon, and that the two cultures are more closely connected than they initially appear.