It is the season to be melancholy, to contemplate, in groaning surfeit, the armature of bone beneath our cope of stretched flesh. The publication of The Undiscovered Country: Journeys Among the Dead, an expedition by Carl Watkins across the ill-defined borders of mortality, is timely. It comes, in smoothly curated form, at just the right moment, between indulgence and regret, on the tide of a new year. When the days are short, we are closest to the medieval world. To the avoidance of mirrors where death improves our portraits every morning with a few more lines and shadows. What would once have been a sermon, a conjuring of hellfire, a phantom slide show, is now an entertainment. But before we can begin again, we have to kick free of the embrace of our inconvenient predecessors, that compost legion of the anonymous dead. They come uninvited, requiring us to sign up for what the late Derek Raymond called the general contract: a brief turn in the light, then extinction. Eternal darkness. How to live with such knowledge? William Burroughs admired the unswerving bleakness of Beckett's gaze, the way he reduced compensatory illusions to zero. Nowhere left to crawl. And nothing to crawl on. Last breath is last breath. Stare into the abyss and the abyss will stare right back.

Taking a strategic sabbatical as a labourer in a French vineyard, Raymond chose to see London as a city of memory, a necropolis; a labyrinth of unreason patrolled by rogue mini cabs driven by chain-smoking romantics (not unlike himself). Interviewed in 1992, about his darkest novel, I Was Dora Suarez, he said: "Death lasts much longer than life. Life really is an illusion. I don't cry for the dead, I speak to them and they speak to me. And that's the end of it. Amen."

We are ventriloquised, reluctantly channelled, by the ones who came before us. And Christmas, Carl Watkins suggests, is the portal, the season of return. "Generations had held that spirits had special power around Christmastide. The ghosts of the glorious dead would, for a day, come home." The unbearable loss of the young men of the 1914-18 War thinned the membrane between the worlds of the living and the dead, even for the most rational and orthodox of parents attempting to come to terms with bereavement. Sir Oliver Lodge, a distinguished scientist, professor of physics at Liverpool University, was attracted by the possibility of communicating with his son Raymond, who died at Hooge Hill, near Ypres. Lodge published Raymond's letters from the Front as well as dispatches from the afterlife. In 1915, a place was set at the Christmas table, in the hope that the officer killed by splinters from an exploded shell would make his presence known. A consoling dialogue was anticipated, a chatty report from the shimmering pastures where souls drift like untethered barrage balloons. The terrible warning of "The Monkey's Paw", the WW Jacobs story about what might actually happen if that wish were granted, and a son crawled from the grave to scratch at the door, was forgotten.

Watkins demonstrates how different areas of Britain arrive at their own tactics for coping with the inevitable: the ecumenical mystery of the afterdeath. A state we all experience but nobody can describe. The Undiscovered Country is weighted as a voyage through time, by way of legends, brief biographies, and character sketches. Haunting myths emerge from specific British topographies. Death and terrain are old intimates. As a medieval historian, Watkins is strong on pre-Reformation eschatology, Catholic England with the Day of Judgment as much a palpable reality as the unpredictable whims of popes and princes. There was a moral road map by which to navigate and it was issued in Rome. Purgatory was a long corridor between the death trauma and an ultimate destination in heaven or hell. Judgment was visible on the walls of churches, sheep and goats divided. For those who acquired some measure of wealth, life was an awkward negotiation against the barefoot pilgrimage through limbo, for which no servants or bearers could be hired. Acts of conspicuous charity were plea-bargains supported by requiem masses or future memorials, before it was permitted to receive God's grace, or to face the fiendishly inventive demons of a Hieronymus Bosch bestiary. Souls had their colours, pitch black to speckled, to pearly grey, as they moved towards a state Scientologists still promote: Clear. Purified by fire. Pre-forgotten. Liberated from the sticky remembrances of survivors. This is the paradox: in honouring our loved ones, we anchor them to the ground where they formerly belonged. "The soul in purgatory was a traveller passing through, not a permanent resident," Watkins says. "For all the horror, there would be release."

Before Protestants abolished purgatory, which had been corrupted by the obvious abuses of the clergy and their accountants, the familiar dead walked the bounds of Suffolk villages, worrying at unresolved sleights, taking revenge on those who betrayed them in life. Death was a settling of accounts. On the North Yorkshire moors there were ghost paths and tracks where grumbling spectres might challenge travellers. Unpurged souls would be hiking through the topography of metaphor  bogs, sucking ponds, dark woods  for years without number. Watkins references a woman called Agnes de Lond who could not rest because she barred her children from their inheritance, leaving land to her brother. "She would walk until he died but then he would take her place. The moment for setting things right would then have passed; he would walk until the end of time."

With the decommissioning of purgatory, it was decreed that the dead went immediately, with the last breath, to heaven or hell. By the time of the English Civil War, church monuments were being smashed as idolatrous; wall paintings were covered with whitewash, brass plaques melted down for profit. Ghosts were downgraded, but they were slow to disappear. "Sin eaters" traded in the literal swallowing of mortal faults, a reverse communion by which those who had grown plump on indulgences and unfettered lusts could be pecked and nibbled to holy essence, wraiths of spirit.

When the soul escaped from a tactfully opened window, the vacated carcass was no more than a husk, a meat machine fit only for the anatomist's knife. Resurrectionists and freelance grave robbers thrived. The bodies of condemned felons, such as John Horwood of Bristol, were awarded to the surgeons. A final shame and punishment for the families of the guilty. "In no case whatsoever," said the Murder Act of 1751, "shall the body of any Murderer be suffered to be buried." Horwood's sad story was recounted in a small volume, which included depositions, trial proceedings, and a sketch map of the crime scene. It belonged to Richard Smith, the anatomist who carved up the hanged man in an act of public theatre. The memorial book was bound, for this discriminating antiquarian, in Horwood's cured skin.

Burial grounds were overburdened. There were 100,000 corpses in the tight Nonconformist oasis of Bunhill Fields, just outside the City walls in Finsbury; among them William Blake, who shared his narrow trench, not with wife and family, but with a clutch of workaday locals, such as Magdalen Collin, an 81-year-old woman from Bethnal Green Road. Bunhill was a bone hill. Bursting graveyards rose above their host churches, mounds of earth or sand casually shovelled over the putrid dead, whose clutching hands would break through earth or leaves. In Bunhill Fields, labourers are reported to have died from breathing the rank effluvia of the plague pit.

The Undiscovered Country has many such tales to tell. Watkins is one of those rare guides who never overstays his welcome. He wears his research lightly as he journeys around the British landscape, teasing out themes and cultural shifts from the particulars of individual lives. Now the psychopathology of our relationship with those intrusive cotenants, the dead, seems to have come full circle. We are saturated, to the point of numbed insensitivity, with images confusing Eros and Thanatos. The newsreader's prim warning alerts us to the stretching of what can decently be shown, from ruined cities and dying fields, in competition with the car crashes and decapitations of the internet sump. Here is JG Ballard's death of affect. A vast CGI panorama returning us, without theology, to the medieval wall painting. Catherine Ballard, in Cronenberg's film version of Crash, contemplates a highway of tangled metal, smoking wreckage and torn flesh. "They bury the dead so quickly," she sighs. "They should leave them lying around for months."

It's that time of year when we make resolutions and big decisions. Big journeys too. But have you made any plans for leaving? And more importantly, do you have any idea of where you will go when you are no longer resident in this vale of tears? Will you snuggle up with virgins under a shady tree beside a cool river? Join the celestial choir up there beyond the clouds? Or how about returning in a different sort of body (a mouse, perhaps, or a king?) for another spin of the wheel of life? Death, and what follows, is something from which we are increasingly removed. And this, in part, explains the fascination of a book that looks at ideas of where we might go when we shuffle off this mortal coil.

Carl Watkins is a Cambridge historian who specialises in medieval Britain, so it comes as no surprise that his story starts not with an ancient man interred with some pots and other pieces and his body curled into the foetal position, but with a man called John Baret. Baret was a prosperous trader who lived in Bury St Edmunds, Suffolk, in the first half of the 1400s. There is no reason why you should know anything about him or his life. In fact, it has taken all Watkins's skills of historical deduction to construct any sort of narrative about him because all he had to go on were Baret's will and his tomb in St Mary's Church, Bury. From what might seem scant material, Watkins has resurrected what could have been Baret's view of life and death.

Baret's success in business was reflected in his will. He lived in a mansion and owned many beautiful objects of significant value. Like most people in 1400s England, he went to church and would have heard the story about the rich man and the eye of the needle. He appears to have believed it, which explains why he built his tomb inside the church during his lifetime and topped it not with an image of himself in perfect state, but with a corpse. Baret then wrote detailed instructions for his funeral, including prayers to be said, people to attend, even what clothes should be worn. He also left money to pay for it all, as well as prayers and remembrances to be said for him through the coming years. His attempt to buy intercession, and through it salvation, suggests that he believed in judgment day and the afterlife. Watkins makes the point that although Baret was unusual in his ability to pay for all this, most people of his time would have shared his beliefs and desire.

Two great shifts in the history of British mortuary beliefs bookmark this tale. The first happened during the Reformation, in 1552 to be precise, for it was at this moment that a new prayer book was issued to make clear the new thinking: that the dead could not be reached, or helped, by prayer, just as the dying could now not be saved by confession. The second big change came with the mass slaughter of the first world war, when many sought refuge from grief in the idea of the glorious dead living on. Between these two moments, Watkins shows how tenaciously people clung to the age-old belief that the soul or spirit had an afterlife, suggesting that communing with the dead is a basic human urge. There was another side to this, what Watkins calls the "supernatural deterrence" of Hell, the threat of whose fires and torments kept many people on the path of good during this life. William Gladstone, for one, believed that fear of Hell played a significant role in maintaining order in society.

Watkins does several things particularly well. He tells a good story, or a string of them spanning the centuries. He makes locations accessible with some very vivid writing about place. But above all, he is good at summoning the spirits of the long gone and mostly unillustrious, from Robert Parkyn, a 16th-century Yorkshire clergyman, and John Knill, an 18th-century Cornish customs man, to Raymond Lodge, who died at Ypres early in the first world war and whose dispatches from the afterlife, received via a medium, were published by his grieving father. These stories  and what they say about our urge to reach beyond the grave  are what make this book so fascinating.

I would have liked Watkins to reach around the world as well as through time. He describes a medieval "doom"  a painting representing the end of time  in which the Archangel Michael uses a pair of scales to weigh the soul of the deceased against their sins. The parallel with the ancient Egyptian idea of judgment before the god Osiris, with the heart of the deceased weighed against the feather of truth, is irresistible. But his focus is here on Britain, primarily on England, which is where he ends. Standing on the threshold of multicultural Britain, Watkins muses on the likelihood of the idea of an afterlife surviving an age where most of us no longer practise a religion and where very few of us have any certain idea about what happens in that "undiscovered country" beyond the grave. At a time when many of us are making resolutions and big decisions, this could be essential reading.