Near the end of Le Côté de Guermantes, the third volume of Proust's À la recherche du temps perdu, comes back from a visit to baron de Charlus and finds a letter by a young footman to a friend lying on the table. He assumes that the footman has forgotten it there before retiring to bed.
The narrator has spent the entire book (i.e. Le Côté de Guermantes) talking about his encounters with the upper classes and the nobility (Madame de Villeparisis, Saint-Loup, Madame de Guermantes, Charlus, etcetera), and about the thoughts that these encounters triggered in him. He has shown little interest in servants and the lower classes in general; Françoise is still present, but plays a much smaller role than in the first two volumes; Jupien is introduced in the beginning of the novel but does not become important (and always remains a secondary character) until Sodome et Gomorrhe. So why does Proust makes his narrator read the footman's letter?
Since Proust's work is now in the public domain, one can find the text of the letter online, for example on ALaRechercheDuTempsPerdu.com and on Wikisource. Wikisource provides a scan and the text of page 223 and page 224 of the 1919 edition.
Dear Friend and Cousin,
I hope this finds you in good health, and the same with all the young folk, particularly my young godson Joseph whom I have not yet had the pleasure of meeting but whom I prefer to you all as being my godson, these relics of the heart they have their dust also, upon their blest remains let us not lay our hands. Besides dear friend and cousin who can say that to-morrow you and your dear wife my cousin Marie, will not both of you be cast headlong down into the bottom of the sea, like the sailor clinging to the mast on high, for this life is but a dark valley. Dear friend I must tell you that my principal occupation, which will astonish you I am certain, is now poetry which I love passionately, for one must somehow pass the time away. And so dear friend do not be too surprised if I have not answered your last letter before now, in place of pardon let oblivion come. As you are aware, Madame’s mother has passed away amid unspeakable sufferings which fairly exhausted her as she saw as many as three doctors. The day of her interment was a great day for all Monsieur’s relations came in crowds as well as several Ministers. It took them more than two hours to get to the cemetery, which will make you all open your eyes pretty wide in your village for they certainly won’t do as much for mother Michu. So all my life to come can be but one long sob. I am amusing myself enormously with the motorcycle of which I have recently learned. What would you say, my dear friends, if I arrived suddenly like that at full speed at Les Ecorces. But on that head I shall no more keep silence for I feel that the frenzy of grief sweeps its reason away. I am associating with the Duchesse de Guermantes, people whose very names you have never heard in our ignorant villages. Therefore it is with pleasure that I am going to send the works of Racine, of Victor Hugo, of Pages Choisies de Chenedolle, of Alfred de Musset, for I would cure the land in which I saw the light of ignorance which leads unerringly to crime. I can think of nothing more to say to you and send you like the pelican wearied by a long flight my best regards as well as to your wife my godson and your sister Rose. May it never be said of her: And Rose she lived only as live the roses, as has been said by Victor Hugo, the sonnet of Arvers, Alfred de Musset, all those great geniuses who for that cause have had to die upon the blazing scaffold like Jeanne d’Arc. Hoping for your next letter soon, receive my kisses like those of a brother.
However, reading this letter in an English translation won't be sufficient to find the answer to my question.