What she saw there was one of the most important collections of letters ever written by an artist. Theo, an art dealer who, better than anyone, understood and appreciated his brother’s talent, had carefully hoarded letters, photographs, bills, account books and other documents relating to Vincent and their family.

After Theo died in January 1891, six months after the suicide of his older brother, Jo kept alive the Van Gogh flame with tenacity and passion. Some 13 years later, the first major edition of the artist’s letters, edited by Jo, appeared, adding lustre to a reputation at last beginning to gather admirers. Rightly, in their introduction to this latest edition of Van Gogh’s letters, its editors, Leo Jansen, Hans Luitjten and Nienke Bakker, pay tribute to the woman who is often reduced to a bit part in the great Van Gogh saga.

“She published only the letters from Vincent to Theo (and to her, after her marriage to Theo in 1889) along with a few family letters,” they write. “She had made a huge contribution to the recognition of Van Gogh by doing her utmost to exhibit and sell drawings and paintings”.

Subsequently, improved and more complete editions of the letters appeared but until today none could claim to be comprehensive or definitive. Now, in six stout and sumptuous volumes, every one of Van Gogh’s 902 extant letters has been reprinted, alongside the paintings by Van Gogh and others that are referred to.

It is a monumental scholarly achievement which not only refreshes our knowledge of Van Gogh but allows us to follow his stumbling path from rookie painter painstakingly copying the works of his predecessors and contemporaries, in order to understand how to pursue his craft, to that of peerless genius whose compositions are simultaneously and instantly recognisable and constantly surprising and inspiring.

What makes Van Gogh’s story all the more fascinating is the way in which he writes about himself, describing his world view, what he’s reading, the galleries he visits, the sunsets, his family, his huge ambition and desire to realise it, and his almost total obscurity during his lifetime.

He had wanted to be a pastor like his father. Only after he was deemed unsuitable in that calling did he turn to art. His Pauline conversion is detailed in the letter to Theo dated June 19, 1879, when he was 26 years old. He outlined his artistic creed, paraphrasing Francis Bacon: “I know no better definition of the word Art than this, ‘Art is man added to nature’, nature, reality, truth, but with a meaning, with an interpretation, with a character that the artist brings out and to which he gives expression, which he sets free, which he unravels, releases, elucidates.”

His first serious painting, The Potato Eaters, came in the spring of 1885, a sketch of which he included in a letter to Theo.

“Herewith,” he wrote, “two scratches [of farm labourers] after a couple of studies that I made, while at the same time I’m working on those peasants round a dish of potatoes again. I’ve just come home from there – and have worked on it further by lamplight – though this time I started it in daylight. See, this is what the composition has now become. I’ve painted it on a fairly large canvas, and as the sketch is now, I believe there is light in it.”

Such a record of an artist’s creative history is unprecedented. Though Van Gogh was obviously a compulsive and natural correspondent, he was also, the editors suggest, encouraged to be so by Theo who, much as he loved his brother, wanted to keep tabs on him, not least because he was contributing to his upkeep. The letters, therefore, as well as being full of family chit-chat, pious observance, biblical quotations and lyrical passages about nature, comprise a detailed record of an artist’s working life.

Having committed himself to his career, Van Gogh embraced it with wrench-like tenacity. Moving from Holland to Paris, where he lived with Theo, he abandoned flat landscapes and dun colours for a new, brighter, happier and characteristic palate. Moreover, in the French capital he was exposed to the Impressionists and Neoimpressionists and met the likes of Paul Gauguin, Camille Pisarro, Toulouse-Lautrec and Emile Bernard, with whom he maintained one of his most formatively important correspondences.

Three letters to artists survive from this period, including, most notably, one to Bernard in which, amid a blizzard of advice, recommendations and gossip, he wrote: “I persist in believing that you’ll realise in the studios not only does one not learn very much as far as painting goes, but not much that’s good in terms of savoir faire, either. And that one finds oneself obliged to learn to live, as one does to paint, without resorting to the old tricks and trompe l’oeil of schemers.”

Paintings now poured from Van Gogh. It was as if he was in competition not only with his peers but with himself and his mortality. In the 10 or so years during which he was truly productive, he amassed a phenomenal body of work – believed to have been more than 900 paintings – with distinct phases and shifts in subject matter, mood and scene.

Inevitably, after two hectic years, Paris began to pall and he moved to Arles in Provence, from which his letters, almost 200, are replete with references to many of his most famous paintings. Supplied with money and materials by Theo – “As long as you can bear the burden of all the colours, canvas, money that I’m forced to spend,” he wrote, “keep on sending them” – he set himself a dizzy schedule.

You can see why. In letter after letter he describes things he must capture and paint. To Theo, he wrote letters that are themselves paintings. “A blue sky with white clouds. An immense field of an ashy lilac, furrows, innumerable clods of earth, the horizon of blue hills and green bushes and small farmsteads with orange-coloured roofs. It’s another of those that’ll take a long time to dry; with impasto paintings you have to do the same with the strongest wine, it has to ferment.”

Reading such joyous sentiments, it is easy to forget the mental turmoil and personal tribulations that dogged Van Gogh throughout his life. Famously, he sold only one painting in his lifetime. Indeed, a few months after writing the above, following an argument with Gauguin, he cut off part of his left ear. “Now let’s talk about our friend Gauguin,” he wrote to Theo, “did I terrify him? In short, why doesn’t he give me a sign of life?”

Van Gogh’s nonchalant response to what was clearly a deterioration in his mental health reads all the more poignantly when the following pages are illustrated by his self-portrait with bandaged ear. Much of the time between this event and his suicide was spent in asylums or recuperating, but, whenever he was able, he was at his easel. During his last two months, say the editors of his letters, “he was painting and drawing with the utmost intensity ... producing an astonishing number of portraits and landscapes.”

His last letter, sent on July 23, 1890, four days before he shot himself in the chest, was to ever-faithful Theo: “Ah well, I risk my life for my own work and my reason has half foundered in it – very well – but you’re not one of the dealers in men; as far as I know and can judge you really act with humanity, but what can you do...” Like Van Gogh’s life, it ends abruptly without a full stop, which somehow seems appropriate.

Vincent van Gogh – The Complete Letters is published in six volumes by Thames & Hudson, £325 until the end of 2009, £395 thereafter. An exhibition of Van Gogh’s letters is at the Van Gogh Museum, Amsterdam, until January 3, then at the Royal Academy of Arts, London, from January 23-April 18 www.vangoghletters.org