The Quality of Mercy, By Barry Unsworth

In 1992, Barry Unsworth's fast-flowing epic of the slave trade and resistance to it, Sacred Hunger, jointly won the Booker Prize. It has taken almost 20 years for a sequel to sail into port from Britain's master-builder of historical fiction. The Quality of Mercy acts as coda and retrospect for the earlier story, at the same time as it sets a new course. Although the book can stand alone, a prior reading of Sacred Hunger will sharpen your appreciation.

With Unsworth, a richly elaborated sense of place, period and character never serves as an end in itself. He writes novels of ideas, however concrete and compelling their venues and epochs. And the idea that drives his latest, with its three plot strands that entwine over the spring and summer of 1767 in London and County Durham, is an eminently 18th-century one: sympathy, and the imagination that it requires.

From courtroom challenges to Atlantic slavery to the cramped coal mines of the North-east, the book sets the suffering of the powerless against the fellowship and solidarity that enable the downtrodden both to endure, and to enlist the reforming conscience of the privileged. "It is the power of imaginin' that makes a man stand out," says the roving Irish fiddler Sullivan, our far from saintly ethical arbiter, who pilfers the few shillings he ever has and merrily redistributes them to innkeepers and whores, "an' it is rarer than you might think, it is similar to the power of music".

We hear this music of sympathy drift across the novel's stages. In London, the remnants of the crew of the Liverpool Merchant await trial for piracy: this is the slave ship whose sailors revolted in Sacred Hunger as its human cargo was thrown overboard. The survivors, black and white, found refuge in Florida until Erasmus Kemp, driven son of the ruined ship-owner, hunted them down and brought them back to face his notion of "justice". Legal wrangles, stand-offs and judgments fill a fair proportion of the book. For all the courtroom tension, dependence on previous, offstage events may mean some readers find them dry.

Kemp spars with the Abolitionist campaigner Frederick Ashton (based in part on Granville Sharp), whose spirited sister Jane he loves. Ashton seeks the freedom of a slave brought to England but then kidnapped by his "owners", while the lost ship's insurers challenge the belief that the drowning of chained slaves can be "lawful jettison". Backdating them a little, Unsworth fictionalises the twin landmarks of the Zong case of 1783 and Somersett's case of 1772. However legalistic the arguments, both causes célèbres heralded Abolition. The novel captures all their dramatic twists.

One of the Florida mutineers, Sullivan has slipped out of jail at Newgate. He makes his way north to tell the family of a dead shipmate, Billy Blair, what befell him. This picaresque progress from tavern to fair to workhouse lets Unsworth unfold a tragi-comic, richly coloured panorama of Georgian England as land enclosure and fledgling industry push the populace out onto the road. In the Durham coalfield, depicted with Unsworth's matchless palette for local speech and custom as much as scenery, the pit-shaft underworld through which the Bordon boys hew calls forth some darkly brilliant passages of prose.

Young Michael Bordon wins a handball match for his employer, the spendthrift Lord Spenton – in whose mines Kemp seeks to invest as the former slaver turns to home-grown pillage (with a virtuoso set-piece picnic in the pleasure-grounds of Spring Gardens, as old acres welcome new cash). Well-meaning, insouciant, Spenton rewards the pitman, and hears out his story. Suddenly, the peer glimpses "what it must be like to toil and hate the toil". It is one of those key moments of kinship and kindness that, as Unsworth shows, may unlock the door into a better moral world.