Truman Capote was a born e-mailer who, unfortunately, was born and died a couple decades too early to realize his gift. In the place of what surely would’ve been examples of electronic communications at their finest – zippy, terse, gossipy, and animated by a frenetic wit – “The Collected Letters of Truman Capote” (Random House, 512 pages, $27.95) instead delivers a trough of epistles that, in contrast to the permanence of the now antique practice of letter writing, are remarkable mostly in their determined, almost pathological breeziness and lack of substance.

What stands out by far the most in this collection, compiled by Capote’s major biographer, Gerald Clarke, is the historic volume of flattery the author heaps upon his pen pals. The secret of Capote’s success as a social phenomenon is horrifyingly unveiled by his endless string of “I love your big brown eyes and your long long lashes,” and “What kin are we all to each other? … All the kin in the world.” A good half the words in his letters are devoted to over-the-top sweet talk.

Eavesdropping on these moments is almost embarrassing, like walking in on some sort of particularly inappropriate seduction, especially when you catch him using the same techniques on different victims. A favorite move when he has doused the plate with powdered sugar is to add an extra dollop of hot fudge by passing on a bit of second-hand flattery, generally with an A-list name attached. “He [William Faulkner] has a picture of you which he carries in his wallet – he likes to show it to people” or “Dick Avedon was in Paris the other day, and we talked on the telephone; we talked about you and he sent his best love.”

Wading through these effusions will be a chore for non-Capote scholars. Explaining the inclusion of so much baby talk, Mr. Clarke writes,

Anyone not better informed would assume that he had carried on affairs with most of the people in this book. But the truth is more interesting, if less lurid. Like a child craving affection, he loved his friends without reservation – he told them so again and again – and he expected from them an equal affection. “I feel full of love for you today,” he wrote Andrew Lyndon, a man with whom sex was never considered; “woke up thinking about you and wishing that it wasn’t Sunday so there would be at least the hope of a letter.” Who could resist such an embrace?

Apparently not Gerald Clarke. He reproduces hundreds of pages of these embraces, in which Capote sounds more like Holly Golightly than one of the great literary observers of our times. Through the first two-thirds of the book, letters flutter by – showering praise, tossing handfuls of famous names like birdseed (Tennessee Williams, Greta Garbo, Andre Gide, and W.H. Auden make cameo appearances, alongside dozens of others), but offering only tiny samples of the lush, descriptive voice Capote was capable of in his writings.

In journalism and stories alike, Capote captured postwar America at its high and low extremes with an unrivaled acuity and humor. Able to paint the tics and nuances of New York society and Southern rural life equally, Capote’s gift for framing a scene, finding the phrase or moment that captured a person or event, was uncanny. His 1956 articles following the Everyman Opera Company’s voyage to perform “Porgy and Bess” in the Soviet Union remain the gold standard of observational journalism, as witty, readable, and brimming with insight.

But, sadly, in his letters, he seems to have granted his pen pals the slightest fragments. One letter sent to David O. Selznick from Italy, for instance, provides this merest hint of dramatic intrigue: “There are some simply extraordinary people in Portofino – the place is rampant with the kind of Goings-on Jennifer [Jones, Selznick’s wife] never really believes Go-on. There is an Australian girl who ran away with her stepfather – and a Swedish mother and daughter who share a fisherman between them, etc. But these are very ordinary instances. Altogether, the place is fraught with peril.”

Or he’ll scatter little moments, like Picasso leaving sketches on cafe napkins, that are the tiniest fragments of Capotettes, offering a nibble of the master’s voice, but nothing more: “Anky came to see me yesterday and we spent a pleasant afternoon together, me propped up in bed, and her leaning against a blue water-silk pillow, drinking and eating fried chicken.”

Perhaps this is what makes reading the letters an uneasy experience. In 40 years of correspondence, we see Capote madly charm, we see him display his gift for storytelling, we are gifted with lightning-flashes of insight, but that is all. In 40 years, so far as Mr. Clarke has found, his only recorded moments of sadness or pathos are for the pain of friends.

Of his own struggles – with a family dysfunctional to the edge of madness, with uneasy romantic relations, and with the estrangement a strange diminutive baby-faced homosexual in 1950s America must have felt – we are shown nothing. Only the void created by the complete absence of these feelings hints at what vast depths must have smoldered in that unmentionable space. After 300 pages, the uneasy sense begins to gnaw that the charm, effervesce, the explosions of endearments are not, in fact, attempts to draw his beloved friends closer to him, but a confidence game concealing the sadness within.

And then came “In Cold Blood.” Approximately the final third of the letters are devoted to the period when Capote’s obsession with the murder of the Clutters consumed him, until the 1965 publication of his masterpiece – his “non-fiction novel” of the murder and its subsequent investigation. The letters depict in sordid detail just how freely Capote danced around the fact/fiction line while writing the book.

He bombarded the family of Alvin Dewey – the Kansas investigator who was the subject of much of the book and a main source – with the brunt force of his charm, complete with relayed flattery, cash gifts for Christmas, and near-pathological name-dropping (the most jaw-dropping example being when he forwarded Dewey a note from Jackie Kennedy thanking Truman for a condolence letter he had written upon the death of her newborn son.) We also see Capote inform participants in the events that he, for dramatic reasons, will be putting words in their mouths, spicing up their language, and promising to portray them – Dewey in particular – as a vanquishing hero.

But Capote also deals with something bigger than the glib theater he arranged around the Clutters’s graves. What starts as a lark – to investigate a Midwestern murder story – devours his attention for five years and he be comes personally involved with Perry Smith, the condemned murderer whom Capote visits in prison (“an extraordinary and terrible experience” he writes Cecil Beaton).

In a series of letters to the condemned Smith, Capote reveals himself as nowhere else, squirming in his chair, fascinated with the killer, torn between wanting to win him over and a macabre sense of the seriousness of the situation. He forces himself, haltingly and awkwardly, to open up to the killer. In light of the chirpy notes that fill his earlier letters, the image of Capote writing these words, is, appropriately, blood chilling:

Dear Perry –

Last night I woke up and suddenly thought: Perry says he doesn’t know anything about me, not really. I lay awake thinking about it and realized that, to a certain extent, it was true. You don’t even know the surface facts of my life – which has a few similarities to yours.

The letters illuminate another element feeding Capote’s imagination as he wrote “In Cold Blood.” During this period, he closely followed the struggles of his friend and former lover, Newton Arvin, a professor at Smith College who, after being arrested for possession of homosexual pornography, was fired and forced to disclose the names of other homosexuals on the faculty, under threat of imprisonment. The letters poignantly show Capote’s preoccupation with his friend’s plight during his “In Cold Blood” journey. The juxtaposition sheds light on the finished work, which portrays the slain family and their community as backward hicks and the murderers as Nietzschean avenging angels.

That Capote identified with Perry Smith is heartbreakingly shown in a letter quoting a poem by Robert Service. Meant to commiserate with Smith’s plight, the poem transcribed by Capote just as surely described the way this infinitely odd, restless, little man saw himself:

There’s a race of men that don’t fit in
A race that can’t stay still;
So they break the hearts of kith and kin;
And they roam the world at will.

His letters trail off abruptly after this. It seems that the flirty effusive charmer died right around the time Perry Smith met his end in the electric chair. Mr. Clarke writes, “He often said that the harrowing years he spent researching and writing ‘In Cold Blood’ changed him inalterably.”

The letters included after this date are filled with cranky complaints rather than whimsical flattery. It seems almost that once forced to lower the mask, to confront what lay underneath, he could never convincingly perform the act again.

Mr. Rushfield, a novelist and Vanity Fair contributing editor, lives in Los Angeles.