Although Albert Einstein was a prolific writer, he did not think of himself as one. "In the past, it never occurred to me that every casual remark of mine would be snatched up and recorded. Otherwise I would have crept further into my shell," he wrote in a fit of frustration to his biographer, Carl Seelig, in 1953. By this time, two years before his death, his archive contained more than 20,000 items, thousands of which were written by him personally. Today, the archive has more than doubled in size.

A physicist, philosopher, humanitarian, pacifist, political agitator and cultural Zionist, Einstein was also a formidable writer, and very quotable. Because he wrote almost exclusively in German, his words have been translated into dozens of languages - though, as everyone knows, much can be lost in translation.

Translators have difficulty reproducing his sentences faithfully because they inevitably need to move the words around, causing them to lose their rhythm. Moreover, some of Einstein's words have been so miserably mistranslated that one can't recognise the original. Different versions can also lead to confusion about what he actually said. Einstein, as is the case with most writers, is best read in his mother tongue.

Professor John Stachel, a former director of the Einstein Papers Project, believes the secret to Einstein's writing style can be found in his comment to an interviewer: "I am the acoustic type. I learn by ear and give by word. Writing is difficult." But his letters flowed easily in a clear stream-of-consciousness - he wrote the way he would have spoken, without pretensions. "I think he heard the words before he wrote them, and only when they sounded right did he commit them to paper," Stachel says. "Many poets operate this way, but I doubt many scientists do." He speculates that this pattern may have stemmed from Einstein's childhood habit of quietly saying words to himself before repeating them out loud, a kind of self-echolalia.

Einstein never became fluent in English, either written or spoken. In his day, the language of science was German, and there was no need for English until he went to the United States at the age of 54. If he was asked to speak formally or to write a letter or an article, he would first write a draft in German and then a colleague or secretary would translate it. Indeed, sometimes a colleague would write a letter in toto, and Einstein would simply add his signature at the end. The most famous example is his letter to Franklin D Roosevelt, warning the American president about the possibility of Germany's production of atomic weaponry at the dawn of the second world war; it was actually written by physicist and inventor Leo Szilard.

Einstein's writings can be grouped in several categories. First, of course, were his scientific writings, which made him famous. There were many of these, generally published in scientific journals to be read by fellow physicists. These writings, which I cannot pretend to understand either in German or in English, are widely reputed to be simply and clearly written. Second, we have Einstein's humanistic and political writings, which reveal his broad range of non-scientific interests. The best examples of these essays, as well as some eloquent eulogies, can be found in his book Ideas and Opinions, which has been in print for 50 years.

Third, Einstein left an enormous correspondence: he seemed genuinely to like writing letters. At 17 pages, a letter to HA Lorentz, a Dutch physicist he greatly admired, is the longest I came across in the archive. Thanks to Einstein's secretaries, copies of the more formal, typewritten letters were filed away, and recipients fortunately preserved many of his informal, handwritten letters. Scholars have used them over the years as the basis for understanding Einstein's personality and behaviour and to chart the path of his scientific thinking.

It is in Einstein's letters, and particularly those to his personal friends Paul Ehrenfest, Michel Besso and Heinrich Zangger, where one can best get a feeling for his caustic humour, his humanity, his impatience, his various passions. He wrote about pacifism ("he who cherishes the values of culture cannot fail to be a pacifist"); women, whom he loved and respected though "certain restrictive parts of a woman's constitution that were given to her by Nature ... forbid us from applying the same standard of expectation to women as to men"; music ("music does not influence research work, but both are nourished by the same sort of longing"); pipe smoking ("contributes to a calm and objective judgment of all human affairs"), sailing ("the sport that demands the least energy"), prohibition ("I don't drink, so I couldn't care less"); war (particularly his exchange on the subject with Sigmund Freud); and appreciation of animals ("I have always eaten animal flesh with a somewhat guilty conscience").

Typical of his humour is a letter to the captain of a ship whose crew adopted a cat and named it Einstein, in which he wrote about his own cat's reaction to the news: "Our tomcat was very interested in the story, and even a little jealous. The reason is that his own name, Tiger, does not express, as in your case, the close kinship to the Einstein family." In a reply to the conservative women's group, the Daughters of the American Revolution, who protested against Einstein's visit to America in 1932 on political grounds, he deadpanned: "Never have I experienced from the fair sex such an energetic rejection of all my advances; if it has happened, it was never from so many at once."

The progress of his marriage can be traced in his letters, too. To Mileva, the lovesick young Einstein wrote in 1900: "When you're not with me, I feel as though I'm not complete. When I'm sitting, I want to get up; when I go away, I'd rather be at home; when I'm talking with people, I'd rather be studying." Shortly afterwards he added: "How was I ever able to live alone, my little everything? Without you I have no self-confidence, no passion for work, and no enjoyment of life." By 1919 the relationship had soured, as a letter to his cousin and future wife Elsa reveals: "[My wife] is an unfriendly, humourless creature who gets nothing out of life and who, by her mere presence, extinguishes other people's joy of living ... No wonder that my scientific life thrives under these circumstances: it lifts me impersonally from the vale of tears into a more peaceful atmosphere." Often he was in-your-face brash and even brutal, as in a memo he wrote to Mileva stating the conditions under which he would remain married to her.

To Marie Curie he wrote: "I am compelled to tell you how much I have come to admire your intellect, your vitality, and your honesty," while, behind her back, he opined that "Madame Curie is very intelligent but as cold as a herring." But he was also self-deprecating: "With fame I become more and more stupid, which of course is a very common phenomenon," he wrote to a friend in 1919, after he had gained worldwide celebrity when his theory of general relativity was experimentally proved to be correct by a British team of astronomers.

Fourth, it is not well known that Einstein dabbled in writing aphorisms and poetry, often writing his dedications in verse and sending lady friends humorous and affectionate poems. Among hundreds of aphorisms to be found in his archive are: "Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world" (1929); "Whoever undertakes to set himself up as a judge of Truth and Knowledge is shipwrecked by the laughter of the Gods" (1948); "The attempt to combine wisdom and power has only rarely been successful, and then only for a short while" (1948); "A forced faithfulness is a bitter fruit for all concerned" (1953, in answer to a woman who wrote him for advice about her philandering husband); "There is only one road to true human greatness: the road through suffering" (1947, commenting on the plight of African-Americans).

His poems are much more difficult to translate. They are lighthearted, and perhaps better classified as doggerel. He would often include rhyming couplets in letters to his friends or scribble them on postcards or at the bottom of a picture of himself. One such ditty that I ventured to translate is on music, written to a friend in 1939:

Even if one loves to play

One's little fiddle night and day,

It's not right to broadcast it

Lest the list'ners scoff at it.

If you scratch with all your might -

Which is certainly your right -

Then bring down the windowpane

So the neighbours don't complain.

Finally, Einstein was a diarist - at least when he travelled abroad between 1922 and 1932. He kept handwritten journals during his trips to Japan, Palestine, Spain, South America, and the United States, recording his impressions of each country and its citizens. Though much in these diaries is written in short notes, the author emerges as someone very interested in his surroundings and grateful for his chance to witness other cultures.

· The New Quotable Einstein, edited by Alice Calaprice, is published by Princeton University Press in May. The Einstein Almanac was published earlier this year