Elio M. Garcia Jr., and Linda Antonsson are superfans turned bestselling authors. Early in their trans-Atlantic relationship, they bonded over George R.R. Martin and his series of novels “A Song of Ice and Fire.” Today, they’re authors of a bestselling book they wrote with Martin, a history of the world Martin created and HBO is depicting in the series “Game of Thrones.”

I met them one day last month in Gothenburg, Sweden, as afternoon turned to evening in the park of Slottsskogen, which aptly means Castle Woods. All the while, their boxer puppy played the role of a direwolf, leading them as much as they led him.

Garcia and Antonsson aren’t the stereotypical superfans: They didn’t come in costume, and they didn’t incorporate any rituals into their wedding the previous month. Yet Martin’s fantastical setting of Westeros has been a part of their world for 15 years, since they launched their fan site, Westeros.org. They work together from their home in Nödinge-Nol, near Gothenburg, and their teamwork is evident throughout the interview as they interject to confirm or build on what the other says.

Early in our conversation, Antonsson elaborated on the role Martin’s work has played in her working and personal relationship with Garcia. “It’s something we’ve talked about on a daily basis,” Antonsson said. “I don’t think there’s a day goes by …”

Garcia: “Reading something or posting something …”

Antonsson: “Which is quite scary when you think about it.”

Garcia: “That’s a lot of years doing the same thing.”

Through their constant work on the site, they connected with Martin, and their collaboration culminated in October with the publication of “The World of Ice and Fire: The Untold History of Westeros and the Game of Thrones.”

It’s a book that’s difficult to categorize — so much so that Publishers Weekly initially placed it on its nonfiction bestsellers list. The New York Times considers it “miscellaneous.” It’s a history textbook about a place that never existed, told by a character who lives in its fictional world.

But then, Antonsson asks, isn’t all history at least partly fictional? “History is, you’re writing the story of what happened,” she said. “It’s not actually what happened; it is your story of what happened. So there’s always an element of fiction to history.”

That skeptical, metahistorical voice is evident from the start of the book. Its primary fictional historian, Maester Yandel — of the order of maesters who are scholars, healers and astronomers in Westeros — is constantly injecting caveats and cautions, alternate theories and unknowables. Is the world 40,000 years old, or 500,000 years or more? Were the tallest giants 12 feet tall, or 14 feet?

Yandel asks, then doesn’t answer, both questions, on the book’s first page of history. This fictional scholar could teach many lessons to historians and journalists about the mutability of nonfictional narrative, including the numbers that appear to lend it solidity.

Yandel’s methods are ahead of the evidence he has at his disposal, Garcia said. The maesters’ “big problem is they know so much that they don’t know. They know that there are so many things we can’t possibly figure out.”

Yandel reserves particular venom for singers: traveling entertainers who set legends to music and whose priority is not accuracy but pleasing nobles. “We probably figure that a singer ran off with a girl he liked or something,” Antonsson said.

Steven Attewell, a recent recipient of a doctorate in history who blogs about Martin’s books, said in an email interview that he liked the “World” book and appreciated how it weighed evidence. “You can see the maesters behind the scenes trying to reconstruct history through archaeological evidence, comparative sources from other cultures, collected folklore, etc.,” Attewell said. “At the same time, there’s also a good deal of bias involved — the desire of storytellers to make wars and kings grander than they might have been, the political loyalties of scholars following civil wars, and so on.”

While philosophically sound, Yandel’s approach hasn’t pleased all readers. One otherwise favorable Goodreads review hoped for “a more omniscient, encyclopedia-like concordance.” Another critic wrote that “it goes on and on to waste our time about a bunch of civilizations that existed thousands of years ago, or not at all, they may just be legends.”

But Yandel is very much a Martin character. In the “Song of Ice and Fire” series, each chapter is told through a different character’s perspective, necessarily a limited and unreliable one. Martin plans at least two more books, so an omniscient concordance could have spoiled plenty that is to come. More importantly, though, omniscience doesn’t exist in Martin’s world. In his dark, destabilizing stories, no one is sure of anything. “There’s something essential to that not knowing that drives a lot of what George does,” Garcia said.

Even what readers know to be true is often buried beneath the falsehoods of the powerful people who write history. For all his commitment to accuracy and skepticism, Yandel is writing at the pleasure of the current king, so his history becomes less reliable and more sycophantic with every passing year.

It’s appropriate that this fantasy world doesn’t have an omniscient historian: Its creator is himself not omniscient. Martin doesn’t always remember what he has already written, so he relies on Garcia to help ensure what he writes doesn’t conflict with his published work. Garcia devoured the first three books of the “A Song of Ice and Fire” series, then organized the books’ information about their fictional world into a concordance, which he uses to keep Martin’s world internally coherent.

J. R. R. Tolkien, author of the trilogy “The Lord of the Rings,” had fully imagined his Middle-earth before publishing Frodo’s heroic story. Martin isn’t Tolkien. Nor is Martin a demographer, linguist or historian — all of which Garcia mentions during our conversation. “He isn’t anything of that sort,” Garcia said. When Garcia and Antonsson first imagined the “World” project, they hoped Martin already knew the whole history of Westeros. He didn’t.

“He’s got a world that’s kind of like a mile wide but an inch deep,” Garcia said.

Garcia digs deeper. “I have the head for trivia and details and all the weird stuff that you learn reading the books,” Garcia said. “I don’t know why, I’m not usually an obsessive-compulsive type, but for some reason I wanted to … get down all of the details in one place.”

Among the questions he has sought to answer: How big is Westeros? (3,050,641 square miles, more than three-quarters the area of Europe, extrapolating from the length of the wall that separates most of Westeros from its northern tip, and counting pixels.) How many people live in it? (About 40 million, based on three methods, including summing the number of soldiers mentioned and figuring out how big the population would have to be to support them all, using age-cohort methods similar to those used by American historian Bernard Bachrach.) What is the currency, and what is it worth? (Some jousting tournament prizes seem to be worth the equivalent of millions of dollars, which Garcia rationalizes by pointing to NFL players’ salaries.)

Antonsson co-writes, organizes, and manages website design and keeps the work on track. She says the history of Westeros and neighboring lands isn’t just trivia for hard-core fans who will consume anything. It’s fundamental to the story. She expanded on this argument in a paper for her course in literature at the University of Gothenburg, in which she wrote, “The past is always haunting the present.”

For the book, Antonsson and Garcia wrote up all they could find about Martin’s creation — from his books and from an archive called So Spake Martin that collects his statements at conventions, in interviews and in correspondence with fans. Then they sent it to him. And they waited.

Many of Martin’s fans are waiting for him to write. Other duties account for some of his time: He has written an episode of the HBO show each season, collaborated with the creators, visited sets and given interviews. Martin has said he wants to finish his series because he has other stories to tell. But he keeps finding stories to tell in Westeros, including a spinoff series of novellas.

Garcia and Antonsson don’t understand fans who have turned against Martin for making them wait for the ending. “George could be walking across the street and be hit by a bus tomorrow, and that’s it,” Garcia said. “The fact that there’s no end, I don’t care. It doesn’t take away all those hours” of pleasure. “Like, if you love it so much, I mean, be thankful that it exists.”

But they did get a bit frustrated waiting for their collaborator to send along his contribution to their book, for which they signed the contract back in 2006. “We were kind of waiting for George, basically,” Garcia said. He added, “It was a very tight timeline in the end. Even though, you know, we had, technically, years.” From what they hear, that’s how it’s worked with all of the “Song of Ice and Fire” books. “It’s one of those things where George is definitely going to be working down to the wire,” Antonsson said.

In the meantime, they avoided any other major commitments. Garcia put off pursuing a doctorate in English literature at the University of Gothenburg, lest Martin suddenly was ready to push the book to completion. Their collaboration with Martin was mostly electronic: Each of them has met him fewer than a half-dozen times.

When Martin finally turned his attention to the untold history of his world, words came pouring out in his favored word-processing program, WordStar. “From somewhere deep in the crevasses of his mind, this came springing out, and it’s just insane,” Garcia said. “He could have written a three-volume book on his own in a few months at the rate he was writing. Thank goodness he didn’t decide to do that.”

Martin could fly through the writing because he was working with few constraints. With much of the history of his world unknown, he was free to fill in the details — including the parts that the singers got wrong. That’s part of the appeal for Attewell, the scholar who blogs about Martin’s work. “The reality of history, especially for pre-modern history, is that the limited sources we’ve got are pretty much all we’ll ever have, barring some discovery of a huge trove of preserved documents,” he said. But to fill in Westeros’s history, “Martin can always write more stuff.”

The book wrapped with a “madcap” dash, as Garcia puts it. Among the late touches was inventing a figure named Archmaester Gyldayn whose previously undiscovered manuscript stood in for Martin’s additions. The authors continued tweaking all the way to final deadline on June 19. Bantam published the book on Oct. 28. Despite the occasional gripe about the unreliable narrator, the book has sold well enough to be a “lifechanger,” Garcia said, thanks to the “incredibly generous” 50-50 deal Martin made with them.

Martin’s next project is finishing the “Winter” book. He has declined most interview requests, including mine, since a brief tour for the “World” project. He could find it tough to write as quickly as he did for that book. In his world’s present day, he has to resolve the stories already set in motion, with eagle-eyed readers like Garcia watching to make sure it all makes sense.

“It’s the sheer complexity of the many strands he’s woven,” Garcia said of Martin. “He jokes, ‘I’m going to have to kill off a bunch of characters, kind of simplify things.’ ”

Antonsson interjects, “No, no. He’s not joking.”

They and other fans worry that if Martin doesn’t write the ending — for his characters and for his books — quickly enough, then the HBO adapters will write it for him. Antonsson is both a purveyor of news about the books and show, and a fan, which can create conflict. “We don’t really know if we can watch this whole season, because being spoiled through the show on things that we’ve been waiting on reading for, you know, a decade or more, that’s really weird and I find that really awkward,” she said.

The show is a major frustration for Garcia and Antonsson — not for the reason I expected. They don’t mind that HBO has created an alternate world by deviating from Martin’s story; Garcia and Antonsson believe the canon is what’s in the books, unless Martin says otherwise. Their objections to “Game of Thrones” are aesthetic. Garcia calls it “absolutely, hands down, the most amazing production that’s ever been done.” But “at the same time I’m just sad that so much of the stuff that we love, in particular … doesn’t seem to be on the radar.” Much of what’s missing is the history that they loved from the series, expanded on in the “World” book, and will see Martin continue filling in as he finishes the books.

But the show that frustrates Garcia and Antonsson also has provided a major boost. “The TV shows have done a lot to bring people to the books,” Garcia said. “They’ve done a lot to bring people to the world of Westeros and to our website.”

Though Martin doesn’t seem likely to finish the series any time soon, his collaborators are starting to think about life after Westeros. They’ve heard from an agency that’s interested in repping fantasy stories they might want to write. Antonsson, 40, is studying for a degree in literature, and Garcia, 36, might do the same. Meanwhile, they’ve built a community around their fictional world, one that has led to several marriages. They didn’t meet over their “A Song of Ice and Fire” love, but it brought them closer together, first in spirit and then in reality, when Garcia moved to Sweden in 1999. They got married on the day “World” was published, 16 years after getting engaged.

They don’t plan to become historians and data collectors for another fictional milieu. They won’t mind if others do. “There’s plenty of series out there that could use people paying such close attention to the world,” Garcia said. Antonsson added, “But you really have to love it. You have to be absolutely taken with it.”