Just in time to rescue flagging New Year’s resolutions (remember those?) to strengthen the body and improve the mind comes Drood, a hefty, harrowing, and often funny novel that uses Charles Dickens’s life as a springboard. The book’s great length — 784 pages — gives it great bulk; by simply toting it around, you’re building muscle. But it’s Simmons’s deep dive into the final five years of one of England’s favorite sons that’s the real payoff. Separated from his wife and embroiled in an affair with a much younger woman, Dickens took his private life so private that even his most avid biographers have been left to do some guessing. Simmons, a master of mining historical fact and then reimagining it, makes gleeful use of this somewhat secretive period.

The story starts with the real-life Staplehurst accident, a horrific rail crash that took place near London in the spring of 1865. Dickens, whose train carriage jumped the track and dangled over a steep ravine, managed to claw his way to safety. He then scrambled down the embankment to where the rest of the train had fallen, in order to help the injured. In those hours he encountered a scene so gruesome, so soul wrenching, he rarely spoke of the accident again.

Simmons picks up where the facts leave off. In his version, Dickens isn’t alone among the screams and cries of the dead and dying. He’s joined by a tall, pale wraith of a man with lidless eyes, two slits for a nose, and an array of short, sharp teeth. It’s Drood, a mysterious figure who tells Dickens he was headed for London, then unsettles the novelist by naming the city’s foulest slum as his final destination. As Dickens and Drood tend to the injured, the author notes that everyone Drood hovers over winds up dead. Before the awful afternoon is over, Drood has vanished and Dickens is obsessed with finding him.

The author seeks out Wilkie Collins, a close friend and the narrator of the book. Though Collins was a well-known playwright and quite a successful novelist in his own right, his fame and acclaim were easily eclipsed by those of Dickens. Simmons plays on this imbalance, turning it into a festering envy. Over the course of the book, Collins emerges as a buffoonish Salieri to Dickens’s ever-more-enigmatic Mozart. Here’s Collins, early on, doing some hero bashing:

I had seen Charles Dickens stuck in a rural, doorless privy with his trousers down around his ankles, bleating like a lost sheep for some paper to wipe his arse, and you’ll have to forgive me if that image remains more true to me than “the greatest writer who ever lived.”

Still, he’s a loyal friend who’s game for the adventure of joining Dickens in the search for Drood. The pair prowl the worst parts of London in the still and dark of night, which Simmons conjures with obvious relish.

We walked between the dark headstones and sagging sepulchers, passing under dead trees and down uneven paving stones on narrow lanes between ancient vaults. I could tell by the spring in his step and the clack of his cane that Dickens was enjoying every second of this. I was concentrating on not retching from the stink and not stepping, in the darkness, on anything soft and yielding.

The writers creep into the secret, reeking world of Undertown, a subterranean maze of ancient crypts and catacombs where Drood makes his home. There they stumble over rotting corpses, fight off a savage band of feral boys, and meet a police inspector who has devoted his career to finding Drood. As in any good thriller, by the time Dickens and Collins emerge into daylight they are in peril, and we are left with more questions than answers.

Though Simmons doesn’t quite nail the ornate and extravagant tone of Victorian literature, he hits the high points of its favorite genres. There’s the dream allegory made famous in A Christmas Carol; the gothic novelist’s relish of ruin, imprisonment, and death; and a liberal use of exaggerated emotion and cliffhangers, as befits the sensational novel, popularized by Collins himself in The Moonstone and The Woman in White. There’s plenty of humor, too, often at Collins’s expense. Here he is, settling down to a meal that mocks him, even as it teaches us a bit about the era’s culinary landscape:

Tonight, I decided to dine relatively lightly and ordered two types of paté, soup, some sweet lobsters, a bottle of dry champagne, a leg of mutton stuffed with oysters and minced onions, two orders of asparagus, some braised beef, a bit of dressed crab and a side of eggs.

We learn a lot about opium, too. Laudanum, a popular medicine at the time, is usually dispensed by the drop; Collins, however, drinks this powerful stuff by the glassful. By the time he graduates to the hard stuff, smoking opium in the seedy dens of Undertown, he has become a most unreliable — and long-winded — narrator.

With a book this lengthy, it’s tempting to speed through the opening chapters and rush to the heart of the story. Don’t. There’s a careful and clever setup for the marvelous twist at the end — a twist so unexpected that it changes everything that came before. As for whether you need familiarity with Dickens to enjoy Drood, the answer is, probably not. But the more background you’ve got, the more fun the book turns out to be. Simmons’s title, for instance, references — and takes great delight in playing with — The Mystery of Edwin Drood, the thriller Dickens left unfinished at the time of his death. Knowing a bit about the politics and mores of the time helps, too.

You can get the basics of the author’s life in Jane Smiley’s biography, Charles Dickens, a quick and easy read. Perhaps most useful is Paul Davis’s Critical Companion to Dickens, which compiles the people, places, and incidents of the writer’s life into a searchable glossary. Dip into either of these and you’ll see what wonderful use Simmons has made of both the biographical material and the period detail.

Dickens died on the same date as the Staplehurst train crash — on the very instant, according to Collins — five years later. Was it the stroke that history tells us ended the celebrated writer’s life? Or was it something far more sinister, as Wilkie Collins insists? What we do know is that Dickens left behind an incomplete manuscript of The Mystery of Edwin Drood, which ends in mid-sentence. Numerous writers have taken a stab at finishing the book, with forgettable results. With Drood, Simmons goes them all one better. By anchoring this dark and intricate thriller to a firm foundation of fact, he solves the mystery even as he expands it and, in doing so, becomes both Boz and Boswell.