Hundreds of Vincent van Gogh’s letters have been combined with the art he discussed in his correspondence in a new publication that paints the clearest picture yet of the thoughts behind the Dutch artist’s work.

A product of 15 years of research, the six volume edition displays 900 letters from and to Van Gogh, including sketches of paintings he was working on as well as illustrations of more than 2,000 artworks.

“For the first time, you read the letters and at the same time you also see the visual world that he had in his head,” said Axel Ruger, director of the Van Gogh Museum.

“As a self-trained artist he was constantly searching and trying to improve himself and that is reflected in the letters.”

Together with the publication, the museum will display some letters, which are rarely on show due to their fragility, so visitors can gain insight into the artist’s work in progress and compare sketches in letters to his paintings.

After studying Van Gogh’s letters so intensely, curator Leo Jansen said they had unveiled a methodical, disciplined side to an artist more commonly thought of as a tormented soul who struggled with mental health problems during his life.

“It appears that he is really a very rational man, not the mad genius that we know,” said Mr Jansen. “He was very aware of what he was doing, he chose his aims and he tried to achieve them step by step.”

Studying his letters alongside his paintings, drawings and sketches also helps to convey the close connection Van Gogh felt art and literature shared, Mr Jansen said.

“Books and reality and art are the same kind of thing for me,” Van Gogh wrote in a letter to his brother Theo in 1883.

“Don’t you think, it’s as interesting and as difficult to say a thing well as to paint a thing,” he wrote in a letter to fellow artist Émile Bernard in 1888.

Versions of the book editions are published in English, French and Dutch, while a web edition also provides new translations into English.

Like many other artists, Van Gogh was penniless and largely ignored until after his death, when he was recognised as a revolutionary whose influence stretched into the 20th century.

A large portion of the correspondence is made up of letters Van Gogh wrote to his younger brother, an art-dealer in Paris, who supported him financially. He would describe what he was working on, and often included sketches of his latest projects.

“His brother Theo, he was his confidant, he wrote to him practically daily, which in those days was normal because that was the most obvious means of communication in the absence of the telephone or e-mail,” said Mr Ruger.

At Theo’s suggestion, Van Gogh moved to Paris from The Netherlands in 1886 to mix with the Impressionists and their successors, later moving down to the south of France where he suffered from increasing mental health troubles.

Driven to despair, he shot himself in 1890 aged only 37 and died two days later with his brother at his side.

The letters, originally written mainly in Dutch and French, also include the artist’s correspondence with other family members as well as contemporary artists such as Paul Gauguin, though only 80 are letters written back to Van Gogh.

“We have a slightly one-sided communication,” said Mr Ruger. “You can call it a tragedy perhaps that very few letters to Van Gogh survived.”