My father wrote me once a week when I was in college. Chitchat, for the most part. “Your Uncle Joe called. . . . Dishwasher went out. . . . Had a nice jog this morning.” Exactly the kind of stuff people post on Facebook now. I read each of his letters exactly once and put it . . . where? That’s what I couldn’t remember in the days and weeks after his death. I went through box after box, hunting for those ancient relics, and when I realized they were well and truly gone, I felt as if I’d betrayed not just my father but the whole point of his writing me in the first place.

For isn’t there a sacred premise behind every letter? That it will be kept and savored as long as there are eyes to read? Then again, how many of the letters we’ve received over the years are still with us? And what has happened to the letters we ourselves cast into the world? Is anyone brooding over those?

Letter writing may be an art, as Thomas Mallon argues in his richly entertaining overview, but it is a highly contingent and perishable ­one—­a bit like the mural that Joyce Cary’s ­half-­mad artist, Gulley Jimson, paints as a valedictory on a condemned church. For a letter to survive, someone must deem it worth saving, and someone must deem it worth passing down. The famous correspondence of Madame de Sévigné, valued as much for its aphoristic pith (“I fear nothing so much as a man who is witty all day long”) as for its insights into the court life of Louis XIV, was pruned and, in some cases, rewritten by her granddaughter. Scottie Fitzgerald would coldly examine her illustrious dad’s notes for “checks and news,” then dump them in her desk drawer. (It was her daughter who later compiled and published them.) Tennessee Williams’s letters to his sometime muse Maria St. Just have been set aside for posterity, but where are the pages she wrote in reply? Did Williams toss them away in a fit of pique? Or did they just vanish into the maelstrom of his life?

Even letters that survive the test of time may face a stiffer test from history. The words of Abraham Lincoln, Franklin Roosevelt, and Winston Churchill will always command attention. But what of a long-forgotten literary critic named Francis Matthiessen, whom we find in Mallon’s book building a romantic life with another man? What of the deaf English seamstress tensely negotiating her future with a tailor? The Oxford language student struggling to remain faithful to her soldier lover on the far side of the world?

It’s to Mallon’s credit that he is attuned to the drama of these seemingly undramatic lives—and to the grim irony that letter writing today thrives most in extremis, among the prisoners and refugees who have been deprived of electronic communication. “Our situations are very different,” an imprisoned dissident writes Chinese leader Deng Xiaoping; “you are at the top of a billion people and I am at the very bottom—but life isn’t easy for either of us. It’s just that I am not the one making your life difficult, while you’re the one making it hard for me.”

That power imbalance is, at least in the context of this letter, neutralized. Addresser speaks to addressee on equal terms. Still, Mallon knows that most of us approach a volume like this not for democracy in action but for the aristocracy of gossip. This he delivers in abundance. H. L. Mencken on Wallis Simpson: “a highly oxidized double-divorcée.” Hannah Arendt on Vladimir Nabokov: “There is something vulgar in his refinement.” Oscar Wilde on fickle Bosie (his former lover Lord Alfred Douglas): “The mere fact that he wrecked my life makes me love him.”

Telegrams, suicide notes, memos, execution-eve manifestoes—they’re all here. Oh, sure, you may mourn the critters who got away: Elizabeth Bishop or Evelyn Waugh or, hell, Émile Zola. (Was “J’accuse,” his open letter defending Alfred Dreyfus, too public a performance?) But you’re as likely to be astonished by how much Mallon has packed into so small a space: Helene Hanff’s transatlantic flirtation with Charing Cross bookseller Frank Doel; Walter Raleigh’s curiously pragmatic and, as events would prove, premature last testament; Sullivan Ballou’s heart-rending farewell to his wife on the eve of Bull Run (almost impossible to read now without the strains of “Ashokan Farewell” in your ear).

If Yours Ever runs more wide than deep, that is at least partly a function of its subject. Letters must often compress a great deal of ore into a small seam—they make a virtue of their own impoverishment—and the best letter-writers are those who strike pay dirt with the least amount of spadework. This is what Mallon does, again and again. He writes of Colette, living her life “as a kind of giant maw.” Of Lord Byron, bent from birth on “becoming an adjective.” Of John Keats: “No matter how hard circumstances press, the bedsprings of his self are available for falling back on; the harder his fall, the more cheerful his squeak.”

I particularly liked Mallon’s take on Philip Larkin, who “craved sooty windows the way others do bright lights” and whose letters illuminate the distinction between happiness and fulfillment. The former may be what one wants, but the latter is what one needs, and as such is much more profound. Philip Larkin’s natural temperament was deeply, depres­singly fulfilled.”

We might question Mallon’s fondness for puns (“Pushkin came to shove”) and his dismissal of John Milton, an advocate for divorce and a free press, as “English literature’s most august and terrifying adherent to convention.” There are moments, too, when the literary worth of a particular writer (Jean Harris, say, or Neal Cassady) is more obvious to Mallon than to the reader. But there is no denying the love that undergirds the author’s labor or the seemingly laborless way in which he calls these dead pages back to ­life.

What kind of life, though? That’s the question that began niggling at me the moment I closed this delightful book. Yours Ever is conceived as a museum for a lost art, and it is not hard to see Mallon as the docent in the cardigan sweater, ushering us into each room and then sending us off into the gloaming of mod­er­nity. “Mr. Jobs’s world,” he calls it. By which he means a benighted land where people have lost all capacity for reflection and “considered exchange.” Where even educated folk are reduced to sending text messages that read, in their entirety, “r u there?” Where “addictive gratifications have replaced the old, slow anticipation of the daily visit from the mailman.”

There is, in short, a reflexive melancholy to Mallon’s self-appointed mission, and I’m not convinced that all his belletristesse is merited. (Then again, waiting for the mailman has always struck me as a dubious pleasure.) When I sift through my past week’s electronic in-box, I find easily half a dozen messages that qualify as letters in every traditional sense. They are coherently structured, written with care and design. They enlighten, they illuminate, they endear. They even follow the old epistolary ritual of signing off (not “yours ever,” but some venerable variant: “yours” . . . “cheers” . . . “all best” . . . “xo”). My e-mail may not ascend to the level of Madame de Sévigné, but then, neither did Madame de Sévigné all the time.

More to the point, these messages would probably never have come my way if the senders had been obliged to take out pen and paper. Indeed, it is the very facility of elec­tronic communication that makes the Luddite soul tremble. When Mallon complains that e-mail has “made the telegram’s instant high dudgeon affordable to all,” it is clear that the access troubles him as much as the dudgeon. Look at me! I’m a belletrist, too! But does the relative ease of an e-mail’s composition necessarily detract from its value? Are postage stamps a bona fide of literary ­intent?

Even in the age of tweets and pokes and blasts, the impulse to bring order to our thoughts and lives persists, and at the risk of sounding like a techno­jingoist, one might argue that technology facilitates this impulse as much as it impedes it. One might even envision a day when the electronic message becomes more durable than the letter, when we no longer have to rummage through cellar shadows for our father’s old notes because our hard drives have tucked them away in some brightly lit corner.

That’s not the story Thomas Mallon set out to write, but with his wit and range of reference, his curiosity and gift for synthesis, he is as equipped as anyone to write it. Let us hope, then, that he hasn’t signed off on the subject completely, that he is even now composing some postscript that will, instead of making a fetish of loss, observe without prejudice as our missives leave the printed page and head in still-unguessed directions.