It begins the moment you recognise the handwriting on the envelope: definitely not a bill, then. You turn it over a few times, just for the joy of holding it - and then you open it, and the day is changed forever. Few things can lift the human heart as effortlessly as a letter from a loved one; but as an unstoppable wave of technology carries us, crashing and foaming, into the 21st century, the gentle art of letter-writing seems to have been well and truly beached. But would it matter, really, if the love letter were to vanish off the face of the earth? Has the old-fashioned thrill of pen and paper not been properly superseded by the excitement of being able to pick up the phone and speak to any beloved at any time, whether they're in the shower or half-way up Mount Everest? Doesn't email, instant, cheap and blissfully simple - no danger of messages getting "crossed in the post" - contain a considerable potential for post-modern passion?

Maybe. Until you see, laid out before your eyes, a series of incomparable love letters written over a period of more than 15 years and tracing, with almost unbearable poignancy, the curves of a doomed relationship.

Soon to be published here under the title Beloved Chicago Man, a collection of letters from Simone de Beauvoir to the American novelist Nelson Algren caused quite a stir when they were published in France last year - not because the affair had been kept secret, for it was well documented in accounts of de Beauvoir's life by both herself and others, but because the tender, vivid and incredibly regular missives revealed a side of the feminist writer whose existence nobody had really suspected.

Taken as a whole - and the hardback book runs to some 575 pages - the letters are an astonishing outpouring of emotion from a woman who is more usually seen as the epitome of the pragmatic intellectual. De Beauvoir met Algren in Chicago in the winter of 1947. She was in the middle of a lecture tour of American universities, and looked him up at the suggestion of a friend. The attraction was immediate and mutual, and when the tour was over she returned to Chicago for a brief visit. This was to be the pattern of their relationship, for Algren could no more bring himself to leave the US than de Beauvoir could have been prised out of Paris; hence the hundreds of letters, in which she tries to bridge the distance between them by sharing the details of her daily life with him, providing a unique commentary on French literary, intellectual and political events of the late 1940s and early 1950s in the process.

If this was all the letters contained, they would be remarkable enough, but the depths of emotion displayed in them is a revelation. Though she chose to write in English as Algren's French was, to say the least, rudimentary, de Beauvoir was aware of her limitations in the second language and often worried that this lack of linguistic sophistication would let her down. Instead it seems to have liberated her, for who would have thought that the somewhat po-faced author of The Second Sex and The Mandarins could really have written, to the taciturn and spiky chronicler of Chicago low-life: "Darling, now it is midnight, I have worked very much today and seen some people and I am back in the little pink room and I just read your letter once more. It happens very often I just should have to copy your own letters to tell you what I feel: `I did not think I could miss anybody so hardly. If I were to hold you just now I should cry with pain and happiness'. I am glad to suffer by you, I am glad to miss you so badly since you miss me too. I feel as if I were you and you were me. You'll believe in me as I'll believe in you, whatever happens, and we shall never feel apart anymore. There will never be anything but love between us. I wait for you, I long for you. Take me in your arms and kiss me and make me your wife once more . . . "

When it comes to pages of postal passion, of course, intellectual considerations go out the window - and the irascibility, or otherwise, of the recipient seems to have no bearing on the matter. In 1912 Stella Campbell declared herself to the notorious grumpy George Bernard Shaw as follows: "No more shams - a real love letter this time - then I can breathe freely, and perhaps who knows begin to sit up and get well - I haven't said `kiss me' because life is too short for the kiss my heart calls for . . .

All your words are as idle wind - Look into my eyes for two minutes without speaking if you dare! Where would be your 54 years? and my grandmother's heart? and how many hours would you be late for dinner? - If you give me one kiss and you can only kiss me if I say `kiss me' and I will never say `kiss me' because I am a respectable widow and I wouldn't let any man kiss me unless I was sure of the wedding ring . . . "

Far from running a mile from such dizzy breathlessness, Shaw - the man who castigated Christmas as too soppy for words - ran straight into her arms. But if every love letter worth its salt aims for this tone of heightened emotion, not all of them achieve it. A disgruntled lover will often take refuge in a fit of the sulks, like the poet Rupert Brooke, who wrote to the actress Cathleen Nesbitt from New York in May 1913; "I'm in a beastly room over a cobbled street where there's the Hell of a noise; and I've been tramping this damned city all day, and riding in its cars (when they weren't too full); and it's hot; and I'm very tired and cross; and my pyjamas haven't come . . . " Or turn coy, like the two lasses who sent this mysterious missive to Philip Stanhope, second earl of Chesterfiield, in 1657; "My Lord, My friend and I are now a bed together a contriving how to have your company this after noune. If you deserve this favour, you will come and seek us at Ludgate hill a bout three a clock at Butlers shop, where wee will expect you, but least wee should give you to much satisfaction at once, wee will say no more, expect the rest when you see . . . "

Not all 17th-century women were so flirtatious. Certainly not Dorothy Stanhope, who must have dazed Sir William Temple with her list of requirements in a husband. "First . . . our humours must agree; and to do that he must have that kind of breeding that I have had, and used that kind of company. That is, he must not be so much a country gentleman as to understand nothing but hawks and dogs, and be fonder of either than his wife; nor of the sort of them whose aim reaches no further than to be Justice of the Peace, and once in his life High Sherriff, who reads no book but Statutes, and studies nothing but how to make a speech interlarded with Latin that may amaze his disagreeing poor neighbours, and fright them rather than persuade them into quietness . . . "

And not all women, by any means, could turn out an acceptable love letter, as Evelyn Waugh's petulant note to his wife Laura from the military mission in Dubrovnik in January 1945 illustrates all too graphically; "Darling Laura, sweet whiskers, do try to write me better letters. your last, dated 19 December received today, so eagerly expected, was a bitter disappointment. Do realise that a letter need not be a bald chronicle of events; I know you lead a dull life now, my heart bleeds for it, though I believe you could make it more interesting if you had the will. But that is no reason to make your letters as dull as your life. I simply am not interested in Bridget's children. Do grasp that . . . "

Simone de Beauvoir's letters to Nelson Algren are, by comparison, a model of generosity, thoughtfulness and tolerance, even when she tackles topics like the difficulties of the long-distance relationship or notes that he has gambled away the money she intended to be saved for one of their precious holidays together. For the reader who follows the course of the affair from its magical beginnings to the wary friendliness of the later years, it is predictable and surprising and terribly, terribly sad. It's hard to imagine such a story ever being told in a series of emails - but if I ever receive an email which reads: "There is a delicate scent in my room. I have before me the second of your lovely veils, and when I press it to my face, I can almost feel the sweet warm breath from your mouth. The violets you picked for me yesterday, which nearly withered in my buttonhole, are now blooming anew, and smell soft and fresh", or words to that effect, I'll eat my words. Heck, I'll eat every love letter left on the planet.

Beloved Chicago Man: Letters To Nelson Algren 1947-64, compiled by Sylvie Le Bon de Beauvoir, is published by Victor Gollancz at £25 in UK