A candle was kept burning, since he lit his powders using a folded paper rather than striking a match. He generally woke ‘for the day’ at nine in the evening, and had his only meal at that time – coffee and a croissant which Céleste would bring to him when he rang.

When he felt well enough, Proust liked to have a friend visit occasionally, as long as that friend followed certain rules: no cigarettes, of course, no perfume. Proust describes Reynaldo Hahn coming in, playing the piano for a little while, and then leaving again ‘like a hurricane’ (or, at other times, ‘like a whirlwind’ – as Proust puts it apologetically in letter 23).

One can pause to listen, via YouTube, to the composition by Reynaldo Hahn mentioned in the note to letter 23, Le ruban dénoué [The untied ribbon], twelve waltzes for two pianos. As performed by two Italian pianists in a lamplit chamber concert in Rome, it is at times dynamic and forceful, more often gentle. The audience, visible in the background, is so absolutely still that for a while you might think the video has been manipulated and they have been frozen visually. From this, you may stray over to Anne Sofie von Otter singing another composition of Hahn’s, a three-minute song. Her voice is liquid, effortless. In contrast to his style of entering and leaving Proust’s building, Hahn’s compositions tend to be calm, balanced and elegant, intimate, moderate in tempos and dynamics, and characterized by uncluttered simplicity, charm, and sentimentality. Hahn himself, though a singer and gifted interpreter of his music, apparently smoked and talked too much.

Many friends have described visiting Proust in the room, among them the writer Maurice Rostand, who says of the room: ‘Everything was left lying around, his aspirins and his dress shoes; books were piled up in pyramids; ties were strewn alongside catalogues, invitation cards to the British embassy lay next to medical prescriptions …’

Céleste herself, in her memoir, remembers that the predominant colour was blue, and the lamp next to his bed cast a green light because of the shade. She describes the room as perpetually dark as night, and densely smoke-filled when Proust had been burning his powders. The smoke from these powders would sometimes drift under the doors and out into the rest of the building, and the neighbours would sometimes complain.

And it has to be said that in other ways, too, Proust was in his own turn guilty, on occasion, of disturbing his neighbours. He describes to a different correspondent how another long-time close friend, Robert de Montesquiou, came by in the early hours of the morning and, in his emotion, kept stamping on the floor ‘without pity’ for the Gagey family sleeping downstairs (whom Céleste in her memoir describes as plutôt des couches-tôt, or ‘rather early-to-bedders’).

Or, several times, Proust, a great lover of music, and not well enough to go out to concerts very often, would hire musicians to come and play for him in his bedroom. Once, at 1 a.m., impulsively and without prior warning, he sent for the Poulet Quartet to come and play César Franck’s quartet. Before beginning, the musicians (in the room lit only by candles) hung cloths over the opening of the fireplace to stop the sound from travelling – though one would think that was hardly effectual. When they reached the end of the piece, at two in the morning, Proust induced them (for a handsome sum) to start over and play the whole piece again.

But Proust was well liked by his neighbours, on the whole, for the same qualities so evident in his letters to Mme Williams: his grace, eloquence, thoughtfulness, sympathy, gestures of gratitude. Evidence of these good relations is not hard to find: an inscription in a copy of his Ruskin translation, La Bible d’Amiens, to his ‘good neighbour’, one Arthur Pernolet, who occupied the apartment above him before the arrival of Dr Williams, and whom Proust knew already before he moved in; the helpfulness of Mme Gagey when the time came for most (or all?) of the tenants of the building to move out – she supplied Proust with the results of her own research into suitable apartments; and the fact that a year after they were no longer neighbours, having scattered to other buildings, Dr Gagey came to Proust in answer to a request for a consultation: oddly enough – perfectly symbolic – he had put something in his ear to block the ambient noise, it had got stuck, and the ear was infected.

As the vast resources of the Internet allow us to walk along a Paris street that is almost unchanged from the 1920s and gaze up at – or even float over – the building in which one of Proust’s friends died, and as it allows us within a few seconds to begin listening to one of Reynaldo Hahn’s compositions, or to leaf through the caricatures of the very popular singer Thérésa, with whom Hahn was infatuated, so it also reveals more of the life of Proust’s building: the wartime activities of Proust’s 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 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 these letters, 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 Céleste, 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.

Gide describes, in his journal, Proust’s style of talking: ‘His conversation, ceaselessly cut by parenthetical clauses, runs on …’

The diplomat and Proust fan Paul Morand enlarges upon this: the Proustian sentence was ‘singsong, cavilling, reasoned, answering objections the listener would never have thought of making, raising unforeseen difficulties, subtle in its shifts and pettifoggery, stunning in its parentheses – that, like helium balloons, held the sentence aloft – vertiginous in its length … well constructed despite its apparent disjointedness; … you listened spellbound …’

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 strictly controlled, in the extended, balanced periodic sentences of his finished, published work (or, perhaps one should say, never quite finished, but brought to a certain point and then ended). Here is an example from letter 23, with Proust’s characteristic paucity of punctuation and his multiple enclosed subordinate clauses: ‘My friend Reynaldo Hahn who for the 1st time in 15 months was returning from the front and who entered in disarray may have occasioned some noise which would so ill have recompensed that which you are sparing me …’

Another example of the spare punctuation can be seen in letter 14: ‘I am quite unwell as I write but I thank you deeply for the letter that has brought me I assure you a vision more enduring than a bouquet and as colourful.’ And, later in the same letter: ‘… not to mention the innumerable “mature roses” of two poetesses my great friends whom I no longer see alas now that I no longer get up Mme de Noailles and Mme de Régnier’ – note the interpolated comment and at least five, by my count, commas missing which would be present in a more standard syntax.

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 letter 13, 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 this collection of letters, of his complex syntax, and lack of punctuation, as well as his colourful and fertile imagination, comes in letter 25, which is mainly devoted to the cathedral of Reims so heavily damaged by bombardment in the first autumn of the war. Here we approach the precision, the rhetorical heights, and the 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 stylized foliage of the forest of stones.’

The acute understanding of psychology and social behaviour displayed so richly in the novel is another continuing thread in the letters, and is especially apparent in letter 21: ‘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 in the letters, as in the continuation of letter 21: ‘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.’ (With, later in the same letter, his comparison of the various noises that surround him to his Lullaby.)

How revealing letters can be, in the era when they were written by hand and rarely copied over, especially not by the suffering Proust, who so often, according to him, had barely the strength or energy to write even a short note.