As the internet allows us to walk along a Paris street that is almost unchanged from the 1920s, so it also reveals more of the life of the building in which Marcel Proust lived: the wartime activities of his downstairs neighbour, the couche-tôt Dr Gagey, who was commended in 1915 for his ambulance service “in circumstances often difficult and always perilous”; and the legacies of the former upstairs neighbour, Proust’s friend Arthur Pernolet, who, after his death, also in 1915, left funds to at least two Paris museums for the acquisition of paintings. Follow every reference in Letters to the Lady Upstairs (his neighbour Madame Williams) – the originals spent many years in the Musée des Lettres et Manuscrits in Paris before being brought to public attention – and Proust’s world opens out before us.

Proust, so very solitary, as he says in many of his letters, and devoting most of his waking hours to his work, was also intensely gregarious and an uninhibited talker. When he was feeling well enough, he talked without pause, and the person he talked to the most, because she was always available, was his housekeeper Céleste Albaret, an intelligent and responsive listener. He often rang for her after she had gone to bed, and she would come as she was, in her nightgown and robe, her hair “down her back”, as she says. He would talk to her for hours at a time, sitting up in bed leaning against two pillows, while she stood at the foot of the bed.

André Gide describes, in his journal, Proust’s style of talking: “His conversation, ceaselessly cut by parenthetical clauses, runs on …” This style, so natural to him in conversation, pours out also in his letters – letters, as his friend Robert Dreyfus put it, “in which he always wanted to say everything, as in his books, and in which he succeeded by means of an infinity of parentheses, sinuosities, and reversals”. It is the same style that is evident, though more controlled, in the extended sentences of his finished, published work (or, perhaps one should say, never quite finished, but brought to a certain point and then ended).

We are told that Proust wrote very fast. This, too, is apparent in the letters, in the sprawling handwriting, in the tendency to abbreviate, in the occasional missing word, and perhaps, though not necessarily, in the missing punctuation. Yet, at the same time, his syntactical agility is always in evidence, as in a letter in which he includes in one fairly short sentence a rather elaborate, and in this case indignant, parenthetical remark (“as I have been accused”) that manages to enclose within it yet another clause (“it seems”): “I have been so ill these days (in my bed which I have not left and without having noisily opened or closed the carriage entrance as I have it seems been accused of doing) that I have not been able to write.” Here he exemplifies, in a rougher, more urgent way, his declaration concerning his published writing that a sentence contains a complete thought, and that no matter how complex it may be, this thought should remain intact. The shape of the sentence is the shape of the thought, and every word is necessary.

Perhaps the most extreme example, in Letters to the Lady Upstairs, of his complex syntax and lack of punctuation, as well as his colourful and fertile imagination, comes in a letter which is mainly devoted to the cathedral of Reims, which was heavily damaged by bombardment in the first autumn of the war. It approaches the precision, rhetorical heights and luscious imagery of In Search of Lost Time (and with a reference to a Ruskin title covertly slipped in): “But I who insofar as my health permits make to the stones of Reims pilgrimages as piously awestruck as to the stones of Venice believe I am justified in speaking of the diminution to humanity that will be consummated on the day when the arches that are already half burnt away collapse forever on those angels who without troubling themselves about the danger still gather marvellous fruits from the lush stylised foliage of the forest of stones.”

Marie-France Pisier, Emmanuelle Béart and Catherine Deneuve in Le Temps Retrouvé (1999). Photograph: Ronald Grant Archive

The acute understanding of psychology and social behaviour displayed so richly in the novel is another continuing thread in the letters: “I always defer letters (which could seem to ask you for something) to a moment when it is too late and when consequently, they are no longer indiscreet.”

And the gentle touches of humour, so prevalent in the novel, also have their place: “Considering how little time it took to do the work on Ste Chapelle (this comparison can only I think be seen as flattering), one may presume that when this letter reaches Annecy, the beautifications of Boulevard Haussmann will be nearly done.”

How revealing letters can be, in the era when they were written by hand by the suffering Proust who so often, according to him, had barely the strength or energy to write even a short note. Unrevised, a letter may show the thread of the thought as it develops: “When it has subsided”, Proust writes, of one of his attacks and then realises it may not subside, and so goes on to add what has just occurred to him: “if it subsides”.

The letters, written over a span of years and in different moods and physical conditions, show different aspects of his personality and character. He may be gracious and flattering: “At least I would have the joy of knowing that those lovely lucid eyes had rested on these pages”; or flowery and eloquent: “My solitude has become even more profound, and I know nothing of the sun but what your letter tells me. It has thus been a blessed messenger, and contrary to the proverb, this single swallow has made for me an entire spring.” Or, in contrast to his poetic descriptions, he may suddenly deploy, with cool adeptness, a metaphor taken from the world of chemistry: “Already I carry around with me in my mind so many dissolved deaths, that each new one causes supersaturation and crystallises all my griefs into an infrangible block.”

We may find his precision amusing, but he seems in deadly earnest

He is meticulous and particular not only in his requests as to when and where his upstairs neighbours might nail shut their crates; but also in describing the nature itself of disturbance from noise (as he continues the sentence): “… since a noise so discontinuous, so ‘noticeable’ as blows being struck, is heard even in the areas where it is slightly diminished.”

And he goes into detail about the effect of noise: “What bothers me is never continuous noise, even loud noise, if it is not struck, on the floorboards … And everything that is dragged over the floor, that falls on it, runs across it.” I think we readers, peering over Mme Williams’s shoulder, may find his precision amusing, but he himself, though so likely at other times to see the humour in a situation, here seems in deadly earnest. Proust’s style, in these letters, is a mix of elegance and haste, refinement and convolution, gravity and self-mockery, marked by abbreviations and mistakes, very little punctuation, and no paragraphing to speak of, or almost none, as he shifts from topic to topic.

My approach to translating this style has been to hew very close to it, not supplying missing punctuation or correcting mistakes, but at the same time trying to retain as much of its grace, beauty, sudden shifts of tone and subject and distinctive character as I could. It was a pleasurable challenge to attempt to reproduce his non sequiturs, his flowery constructions, his literary references and his meticulous instructions for lessening the intrusions of noise. One is bound to feel compassion – as his neighbours did – for the beleaguered Proust, pushing ahead, against all odds and in the worst of health, with his vast project; it is certainly impossible, in any case, for anyone with neighbours to blame him for being so fussy about their noise.

Marcel Proust’s Letters to the Lady Upstairs, translated by Lydia Davis, is published by 4th Estate. To order a copy for £8.50 (RRP £10) go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.