"Ici Londres … Yvette likes big carrots …Paul has some good tobacco … the secretary is very pretty … I want to be a godfather … and the big blonde is called Bill."

Thankfully, it all made sense to certain listeners tuned into the BBC in occupied France in the early 1940s, conveying that weapons were to be dropped, by parachute, somewhere the resistance could find them. Other broadcasts from London contained similarly coded messages.

Then finally, nearly 70 years ago, the airwaves from across the Channel included seven words from Paul Verlaine's poem Chanson d'Automne: "Wound my heart in a monotonous languor," said the voice. It was one of the signals that the Normandy landings were about to begin.

As France prepares to commemorate what may be the last major anniversary of D-Day attended by significant numbers of veterans next month, a book to be published on Monday examines the role of BBC French-language broadcasts to occupied France. Je vous écris de France, compiled and annotated by the French historian and radio presenter Aurélie Luneau, is a collection of 200 of the 1,000-plus letters that escaped the German and Vichy government censors to arrive at the BBC in London between July 1940 and August 1944. Many featured in a daily news and morale-boosting programme called Les Français parlent aux Français (The French Speak to the French).

With France occupied by the Germans and General Charles de Gaulle in London calling for his defeated compatriots to continue the struggle, the programme was seen as an antidote to Nazi propaganda and for diffusing messages and codes to the resistance, and the letter writers, almost all anonymous for fear of retribution, sent their letters never knowing if they would arrive. Many were censored, some lost. Other writers found the Gestapo knocking at their door. At one point the BBC was receiving 100 letters from France every month.

The book is based on unpublished correspondence packed in nine boxes at the BBC's archives in Reading. Luneau, who rediscovered the cache, says they are a testament to the everyday courage of French people. "Nobody knew about these letters until now. The aim of the book is to pay tribute to the writers, who at best risked having their radio taken away and a fine and at worse, prison, forced labour or deportation to the death camps," Luneau told the Observer. "During the occupation, the French turned to the BBC for their news. France was under the German boot but people knew the BBC would tell the truth even if the news was bad, and this countered German propaganda. With the BBC, people knew they were hearing the truth.

"Before the war 6.5 million people had radios out of a population of 40 million, but people listened en famille or with friends, and those who couldn't listen heard about things from word of mouth.

"Even sending the letters was a challenge. No letters were allowed to be sent from the occupied zone, and even in the free zone they were censored. The BBC has more than 1,000 letters of which there are 200 in the book, but we have no idea how many never arrived, were lost or not kept. We also have no idea how many people tuned in, but the BBC became a conduit for civil resistance. The radio would tell people to dress in colourful patriotic clothes and take to the streets to demonstrate and they would, up to 150,000 of them."

Je vous écris de France, published by L'iconoclaste, opens with a missive from a woman in Béziers who writes: "My dear English friends, thank you for the comfort your programmes bring to those French people who remain desperate for liberty, who do not accept being eaten in Hitlerian sauce, who keep in their hearts, with impotent rage against our bad masters, the tenacious hope of relief."

Men, women and children wrote. Some to praise, others to criticise Britain's lack of military support, or lambast De Gaulle for dividing France. "You have to deliver us, and above all do it quickly," said one letter in childlike writing. "We are dying of hunger and will be shot if we don't obey."

On a sheet of lined exercise-book paper a letter writer signing himself "a young French boy of 13 years" drew the tricolour and the union flag above the caption: "Vive les Allies."

Some wrote of their hatred for "les Boches", others ranted in despair and bitter disillusionment with the British. "The English don't want to fight, they prefer to let others do it … the forces that Britain sent to France in 1940 were extremely weak, badly trained and without the necessary equipment. After that, it was mathematically impossible for a badly armed, badly equipped and very demoralised France to repel more than 200 better armed and trained German divisions … the French have lost all hope in an English victory," wrote one angry listener from Marseille on 29 July 1940.

He continued: "She [England] has no army, her navy is powerless against the German and Italian planes … this is what happens to selfish countries who feel safe at home and make others fight when they themselves don't want to … England stopped France organising its victory in 1918 and allowed Germany to rise again for fear of a France that was too strong …she provoked the war in 1939 and abandoned France in the middle of the battle." The letter finishes: "ps. Your radio is grotesque and insulting to the French."

"There's a whole range of opinions, feelings and emotions," Luneau said. "But for most people during those years, the BBC remained the voice of truth, freedom and hope."