Author Sara Sheridan's voyage of discovery

I wanted to write a novel about James Wellsted as soon as I was shown the three letters he wrote that are now in the National Library of Scotland's John Murray Archive. They dated back to the 1830s, when Wellsted was a lieutenant in the East India Company's Bombay Marine charged with exploring the southern coast of Arabia. I'd already read up on Wilfred Thesiger and Richard Burton - both more famous men who later travelled extensively in the area - but what Wellsted's story had over theirs wasn't just that he'd beaten them to it, but that, even though he was viscerally opposed to slavery, he'd travelled there disguised as a slave trader.

I started to read and remember thinking that if nothing else, I was set to learn a lot about camels. There was a lot of material both in the National Library of Scotland and in the British Library in London. I examined the archival evidence of the Bombay Marine's mission to the Red Sea on which Wellsted served and I tried to find out about what life would have been like at his Marylebone home. I poked at the history of the burgeoning British Empire and gathered information on what the 1830s were like in the countries that are now Ethiopia, Eritrea, Egypt, Yemen, Somalia, the United Arab Emirates and Oman. I read Wellsted's accounts of his desert travels and pondered over the gaps he left in his story, for these gaps are almost as fascinating as what he wrote. A young officer might not want to mention his true love or his deepest fears but in an age before modern psychology there are intriguing clues in what he's written.

During the Edinburgh International Book Festival last summer I heard radical historian Ilan Pappe proclaim: "Don't ever believe an historian who says they don't make up anything. Every historian has an agenda." The more I work with original documents I realise that's absolutely true. It would be difficult, of course, to misinterpret a legal agreement - a treaty or an act of Parliament - but social and cultural history is often comprised of whatever diaries and letters remain and that is down to chance and wide open to interpretation.

I've always been very clear that my job is to write a story rather than to reconstruct actual events. Not all novelists agree. This summer I turned down a commission to dramatise an explorer's notebooks. I thought the resulting document (upon which the publisher was keen) would never be as riveting as the notebooks themselves unless I was allowed to add something of my own. For my money, a novelist has to add value to a story. In Britain we have some extraordinary archives, and much of the material is effectively undiscovered. Thousands of documents haven't been read since they were archived and yet they contain gems. I am frequently moved by the bravery of adventurers whose only memorial is an entry in a ship's manifest or an institutional payroll and whose death is marked by the clerk's removal of their name when the documents have been updated. For every Nelson there were thousands of seamen whose day-to-day acts of bravery go unrecorded in mainstream history. These people, however, make up the flavour of the times - a hugely attractive era of bravado and foolhardy exploration that brought extraordinary rewards to our culture at the same time as sacrificing some of our brightest and best. The average life of a plant hunter, for example, was counted in months, not years, from the moment they set sail. And yet they went in their hundreds. It's this I wanted to catch more than a catalogue of dates or achievements. As Edith Wharton put it, "Slavish accuracy must necessarily reduce the novel to a piece of archaeological pedantry instead of a living image of the times."

This touches on one of the key issues for historical novelists - the thorny matter of accuracy. Secret of the Sands belongs to the genre that Truman Capote named "faction". Writers in this genre work on the borderline between what's real and what they have constructed. While what I write is always largely consistent with the records that remain, I freely admit that where historical fact proves a barrier to invention, I simply move a detail a little one way or another or make up a new story to get my characters out of a fix as rambunctiously as possible (history at its best is a gritty, dirty business).

Muscat in the 1830s was an exotic place full of temptations. Like many of the locations I've written about, that place doesn't exist any more. I wanted readers to taste the sweetmeats and smell the bazaar. I wanted them to understand what it was like to be taken from your home and sold in the marketplace. Or to set off at the age of ten from London and live on a naval ship in the farthest reaches of the globe. For me, that's the imaginative power a novelist has to bring to history. That's the wonder of the job.

• Sara Sheridan's novel Secret of the Sands is published this week by HarperCollins, 7.99. She lives in Edinburgh and tweets as @sarasheridan.