The Queen, God Bless Her, pranced into Bumpers’ shop and bought a copy of Swallows and Amazons. She paid cash for it - I asked.”

So ends a letter to his beloved mother, Edith, from the children’s author Arthur Ransome in 1930. Swallows and Amazons, the first of his children’s adventures, had just been published and the sequel, Swallowdale, was well in hand.

Since then millions of children have joined Queen Mary and enjoyed what Ransome called “the fun of outdoor doings” with John, Susan, Titty and Roger - not to mention their bloodthirsty battles with the fierce Amazon pirates Nancy and Peggy across Wildcat Island.

Swallows and Amazons, and its 11 successors (one every Christmas, as his publishers urged), made their author one of the most successful children’s authors ever. By 1948 the books had sold a million copies (Swallows and Amazons alone accounting for half a million).

Today Ransome’s books, written half a century ago, seem to belong to another world - in which children roam freely at will across deserted countryside, acting out their make-believe adventures, unfettered by adult (“native”) interference. Yet the books’ popularity remains undiminished - publishers Jonathan Cape say all 12 titles currently sell between 200 and 300 copies each month worldwide.

Nor is interest in their author abating. In recent years works have appeared on his sailing, his fishing, his life. Incomplete novels have been unearthed and published, some by the Arthur Ransome Society, formed in 1990 “to bring together all who share the values and spirit he fostered in story-telling”.

Now, next month, comes a collection of Ransome’s letters, selected by his biographer Hugh Brogan, professor of history at Essex University. Illustrated by Ransome’s own drawings, they promise to shed new light both on the man himself and on the people who inspired his stories.

There are letters setting out Ransome’s theories on children’s writing, extracts from a desperately sad correspondence with his daughter, Tabitha, and missives from the period he spent covering the Russian Revolution for the Manchester Guardian. There is also a wonderfully misguided letter from his second wife, Evgenia, urging him not to publish Picts and Martyrs on the grounds that it was a terrible book. Luckily, Ransome ignored her advice.

It was in Russia that Ransome met Evgenia Shelepina, who was then Trotsky’s secretary. As a foreign correspondent he sent back reports which brought him a dangerous reputation as an apologist for the Bolshevik regime. Inevitably, the Bolsheviks suspected him of being a British spy; the British thought he was a Bolshevik agent. At one point, back in Britain, he was arrested by Scotland Yard - but released when he convinced his interrogator he was only interested in fishing. Eventually he and Evgenia left Russia for good and settled in the Lake District, Ransome delighted to be finally rid of politics, which, he told his young daughter in a charmingly illustrated letter showing “Dor-Dor” (Ransome’s pet name for himself) violently throwing up, made him feel quite sick.

In his biography, Brogan reveals that John, Susan and, especially, Titty and Roger, were partly based on the offspring of Dora Collingwood, a friend to whom Ransome had proposed marriage. Dora, however, married Ernest Altounyan and had five children - Taqui, Susan, Titty, Roger and Brigit. Although they lived in Syria, in 1928 the family took a holiday and Ransome spent a happy summer with them sailing two dinghies, Swallow and Mavis, in the Lake District.

Then, says Brogan, “came one of the most famous incidents in the history of children’s literature”. Altounyan gave Ransome a pair of red Turkish slippers. Hugely pleased, Ransome wrote, in return, a story about the dinghies for the family.

But although there are clear comparisons between the adventures and personalities of the Altounyan children and their fictional counterparts, Brogan insists that the characters developed beyond their real-life originals, just as the places in his books are a blend of “the geography of Coniston and Windermere”.

“He fictionalised the Altounyans pretty thoroughly in Swallows and Amazons, ” says Brogan, “and from Swallowdale on the characters began to create themselves.” So much so, in fact, that when Ransome went to stay in Syria with the family he noted in a letter to Edith that his own (fictional) children seemed much “solider” than their real-life counterparts, who were, nonetheless, “very nice and eager to know ‘what is going to happen to us next?’.”

But if there is agreement about the Swallows, the origins of the Amazons has remained disputed territory. Pauline Marshall, for instance, has suggested she and her sister might be the models for Nancy and Peggy Blackett. Ransome saw them playing in the Lake District. But this theory is contradicted by what Brogan calls “one of the real finds” of the collection - a letter from Dora Collingwood’s father.

“Somehow,” writes W G Collingwood, “you have made your Titty so very like my Titty and in a degree your Ruth-Nancy is more like my Ruth than could be expected.”

“Now we know,” says Brogan, “why Nancy Blackett was named Ruth in Swallows and Amazons,” (she changes her name when her uncle Jim remarks that Amazons are ruth-less). She was based on Collingwood’s granddaughter by another of his children, Robin.

In many ways, says Brogan, Ransome retained a childlike outlook for most of his life. “Children make friends, they have their gangs. Arthur Ransome had his gangs. All the time he was writing his children’s stories there were children around.” The Altounyans grew up and moved on - but there were replacements, other children who messed about on Ransome’s boats.

Brogan has a theory that the books dried up when Ransome’s access to children ceased with his move, during the Second World War, to Coniston. “Around then he stopped writing the books. It may be a coincidence, but considering how important children had been to the creation of the books - a suggestive coincidence. Being around children kept alive his sense of what children are like.”

All the sadder, then, to read the letters between Ransome and Tabitha, the daughter from his first, unhappy marriage which was finally dissolved in 1924 and from which Ransome escaped to Russia when Tabitha was just four. On receiving a copy of Swallows and Amazons from her father, she wrote that it “lacked spontaneity and read as if it had been ‘tried after’.” She could not get beyond the first 12 chapters but liked the artist.

Ransome’s response was equally petty: “Your letter in reply to my sending you that book seemed to me deliberately cruel. You told me that my book was ‘churned out’ and tired and yet you must have known that the only time I have had for writing it was time snatched after doing masses of uncongenial work in order to earn enough money for your mother and you.”

Brogan says that when he met the adult Tabitha she looked again at the letters from her father and said: “What wonderful letters, how could I not have loved him, but I did not” - feelings Professor Brogan attributes to the poisoning of the father-daughter relationship by Tabitha’s mother, Ivy. “Even when Ransome invited her to come and stay with him and Evgenia in the Lake District her mother dissented, saying he would drown her.”

Tabitha never went. “She also deeply resented his having deserted her, ” says Brogan. She does not feature in any of Ransome’s successful children’s books.

The letters also contain Ransome’s rather highbrow theories about children’s literature. He believed it was important that children should be given only good books to read. His mother, he says, kept bad books from her children so that when, later in life, he came across rubbish he knew it for what it was.

To Pamela Whitlock, herself a children’s author, he wrote: “Children . .. have no standards. Give them what you want, and if you are the right editor you will find that they want it too. Good books are not addressed to the readers but are overheard by them.”

Ask Hugh Brogan why he has spent 20 years of his life on and off researching Ransome’s life and work and he confesses that he too was a Ransome fan as a child. But the real inspiration came from a review of the film of Swallows and Amazons he read in 1973, which described Ransome as “a frightful old Tory”.

“Of course everyone was to some extent a Tory in those days - but considering he married Trotsky’s secretary and covered the Russian Revolution for the Manchester Guardian I thought this was a bit much,” says Brogan. “He was obviously a mystery to the public and as a professional historian I thought it was my duty to set the record straight.”

And when it comes to setting the record straight, perhaps the last word should go to Ransome himself. In response to a review complaining that his books were written only for “children of the rich”, he replied: “I should be very sorry indeed to think that only children of one particular background can share the fun of open-air doings and the feelings that have been common to all young human beings from the beginning of time.”

Signalling from Mars: the letters of Arthur Ransome will be published by Jonathan Cape on March 27, Pounds 17.99