Simon Sebag Montefiore is sure of one thing. “There is nothing”, he says, “as immediate as letters. Nothing as authentic and visceral as a letter, often written in the heat of the moment, a moment of absolute truth”. And the passion of Sebag, as he is universally known, for letters has translated into an inspired idea for a non-fiction book, Written in History, in which the letters of the famous, the not-so-famous, and the people you have never heard of, march through the pages, bringing their writers brilliantly to life.

“There are so many tones of letters,” says Sebag. “There are diplomatic letters, public letters, love letters, erotic letters. They let you hear how famous people spoke to each other in private.”

It is clear that Sebag has had the greatest fun in picking his selection of 104 letters, with a considerable emphasis on two of his passions — Jews and pre-Soviet Russia.

They are arranged not historically but in categories, such as “love”, “decency”, “war”, or “goodbye”.

Under “downfall” we find one of Sebag’s favourite letters, from the Caliph who ruled Spain in AD 961, Abd al-Rahman III. “It was written to his sons. He was the most successful warrior king in Europe. When he was dying, he left a letter for his children, saying that despite all his plunder and victory, essentially he had had 50 years of glory, but only 14 days of real happiness. I rather love that: there’s something rather Jewish about that attitude, too”.

Tthe book is littered with letters from and about Jews, perhaps the most famous of which is Emile Zola’s 1898 J’accuse, the letter which broke the Dreyfus case wide open. It is, says Sebag, “a wonder of moral outrage”, written to the president of France, Felix Faure, to expose the hypocrisy of the antisemitism which had sent Alfred Dreyfus to prison for allegedly selling military secrets to German intelligence.

Dreyfus was framed and Zola himself suffered for writing the letter, being tried for libel and forced to flee to London.

Unexpectedly, Sebag says: “We need another J’accuse here, today.” Would he write such a letter himself? “I would if I had to. I wouldn’t put myself on the same level as Zola, but I am talking to many other Jewish writers and business people, and everyone is having these conversations: what do we do? It’s not enough to be talking on Twitter or writing to The Times.”

He is swift to praise “an amazing number of people, I would call them Righteous Gentiles, they are really brave people who would stand with us and understand what the stakes are here. Those people are heroic and so are the MPs who are on the front line.”

But he goes further. “When I talk to people about leaving England, we’re doing it with what Jeremy Corbyn would call British irony — and what Jews would call Jewish gallows humour. And also a real optimistic belief in the decency of the British people, that we will never have to leave”.

Nevertheless, as a historian Sebag has a stark warning. “What I call the Corbyn factor is not just about Corbyn. We’re talking about a movement here, tens of thousands of people, organised people, a faction of Leninists.

Leninists understand organisation — and they have taken over the Labour Party. The only way we will defeat them is by some sort of organisation. We, too, have to get organised.

“What it means is that for the first time since 1945, we — Jews and non-Jews — are going to have to fight all over again for the definitions of decency that were accepted after the war. I’m ready to do that, I’m up for the fight.

I’m certainly not going to leave England unless it becomes an impossibly hostile environment. England means everything to me. I was brought up to be so proud to be Anglo-Jewish. We cannot allow this discourse to become normal and acceptable, word by word, inch by inch, neo-medieval antisemitic tropes in public life”.

Zola’s letter, he says, “is so relevant today. The idea of public intellectuals fighting back against racism and antisemitism in a very civilised and sophisticated society… everyone should read this letter. And some time, somebody’s going to have to write one here”.

We return to the book, where there is a letter from someone perhaps unfamiliar to non-Jewish readers, but should certainly resonate with Jews. He is Simon bar Kochba, chosen by Sebag because “it’s great fun to have some really ancient letters in the mix. He was a prince of Israel, known as the son of the star, said by some to be the Messiah. He led the rebellion against the fairly dreadful Hadrian who wanted to build a pagan temple on the site of the Jewish temple — and wanted to rename Jerusalem after his own family, which he did”.

We don’t know much about Bar Kochba, a “bit of a hero” of Sebag’s, but in 1960 a cache of 15 letters from him was discovered in a cave in the West Bank. Written in 135 CE, Sebag says that the correspondence, to an unknown commander of his, “probably dates from the last days of the revolt as the prince struggles to keep control over his diminishing forces. The downfall of the last Jewish state (until 1948) is close”.

The letter is from Shimon ben Kosiba to Yeshua ben Galgoula “and the men of the fortress”. It is succinct: “Peace to you. Heaven is my witness against me that unless you destroy the Galileans who are with you down to the last man, I shall, as I did to ben Aphul, put fetters on your feet”.

Many of the letters are “old friends” to Sebag, who chose them by walking round his library with his daughter, Lily. “She had a camera and I would go up a ladder and fish the books out. But most of them are letters I discovered for myself while doing other research.”

At least two of the letters are guaranteed to make the reader cry: one is from another of Sebag’s heroes, Leonard Cohen, to his long-time muse, Marianne. She was the inspiration for his iconic song, So long, Marianne, and when, in July 2016, he heard she was dying, he wrote to her: “Dearest Marianne, I’m just a little behind you, close enough to take your hand. This old body has given up, as yours has too, and the eviction notice is on its way any day now. I’ve never forgotten your love and your beauty. But you know that. I don’t have to say more. Safe travels, old friend. See you down the road…” Cohen died just four months later.

The other, possibly the most poignant of all the correspondence, is from the Czech Jewish prisoner Vilma Grunwald to her doctor husband, Kurt, written in July 1944 as the family entered Auschwitz. The couple had two children, John, who was disabled, and Frank. When the Nazi Josef Mengele sent John to the left, Vilma chose to stay with him and moments later mother and son were sent to the gas chambers. She hastily scribbled a note to her husband and begged a guard to give it to him. The letter — and Kurt and Frank — survived.

That, believes Sebag, “is the most extraordinary letter in the whole book”. But there are so many more — letters from Hitler to Mussolini, from Stalin to his daughter, from Balfour to Rothschild, from Balzac and Mozart and Pushkin, from Catherine the Great to Prince Potemkin. The selection is overwhelming.

But, inevitably, there came a point when the publisher had to stop Sebag. Just the same, he grins and says he has great correspondence to add to the paperback edition of the book — “letters between Karl Marx and Engels when they were both living in London. They’re absolutely outrageous, but I didn’t find them in time. They are writing to each other in the most obscene, racist way, about Jews, blacks, everything.”

They certainly fulfil Sebag’s definition of a perfect letter: “One that shows a secret side to a person, or beautifully or simply captures a magic moment”.

Written In History, Letters That Changed The World by Simon Sebag Montefiore is published by Weidenfeld and Nicolson, £14.99