Is it a lost art, buried under the avalanche of texts and phone calls and skypings and e-mails? It has certainly become rare. Perhaps it will never again be common. But I think that the wise will not wholly abandon it.
A pleasant letter I hold to be the pleasantest thing that this world has to give. It should be good-humoured; witty it may be, but with a gentle diluted wit. Concocted brilliancy will spoil it altogether. Not long, so that it be tedious in the reading; nor brief, so that the delight suffice not to make itself felt. It should be written specially for the reader, and should apply altogether to him, and not altogether to any other. It should never flatter. Flattery is always odious. But underneath the visible stream of pungent water there may be the slightest under-current of eulogy, so that it be not seen, but only understood. Censure it may contain freely, but censure which in arraigning the conduct implies no doubt as to the intellect. It should be legibly written, so that it may be read with comfort; but no more than that. Calligraphy betokens caution, and if it be not light in hand it is nothing. That it be fairly grammatical and not ill spelt the writer owes to his schoolmaster; but this should come of habit, not of care. Then let its page be soiled by no business; one touch of utility will destroy it all.
If you ask for examples, let it be as unlike Walpole as may be. If you can so write it that Lord Byron might have written it, you will not be very far from high excellence.
But, above all things, see that it be good-humoured (Anthony Trollope, ‘The Bertrams’, chapter 18).