Ernest Hemingway was not only a commanding figure in 20th-century literature, but he was also a pack rat. He saved even his old passports and used bullfight tickets, leaving behind one of the longest paper trails of any author.

So how is it possible that “Ernest Hemingway: Between Two Wars,” which opens on Friday at the Morgan Library & Museum, is the first major museum exhibition devoted to Hemingway and his work? It could be simply that no one thought of it before. Most of Hemingway’s papers are at the John. F. Kennedy Presidential Library and Museum in Boston. After Hemingway’s death in 1961, President Kennedy, a fan, helped his widow, Mary, get into Cuba and retrieve many of his belongings there. Partly in gratitude, she later donated Hemingway’s archive to the new presidential library. But the Kennedy Library, where this exhibition will travel in March, is not accustomed, as the Morgan is, to putting on big crowd-pleasing shows.

Even at the Morgan, Hemingway was something of an afterthought. Declan Kiely, the museum’s head of literary and historical manuscripts and the show’s curator, said recently that he and Patrick Milliman, the director of communications, began idly talking about Hemingway in 2010, after concluding that an exhibition about J. D. Salinger, who had just died, was probably not feasible. The Hemingway exhibition, mounted on walls that have been painted tropical blue to suggest his years in Key West and in Cuba, takes him all the way from high school (where one of his classmates described him as “egotistical, dogmatic and somewhat obnoxious”) to roughly 1950, when he turns up as a self-caricature in Lillian Ross’s famous New Yorker profile. But the largest and most interesting section focuses on the ’20s, Hemingway’s Paris years, and reveals a writer we might have been in danger of forgetting: Hemingway before he became Hemingway.

The exhibition does not fail to include pictures of the bearded, macho, Hem, the storied hunter and fisherman. He’s shown posing with some kudu he has just shot in Africa and on the bridge of his beloved fishing yacht, the Pilar, with Carlos Gutiérrez, the fisherman who became the model for “The Old Man and the Sea.”

But the first photo the viewer sees is a big blowup of a handsome, clean-shaven, 19-year-old standing on crutches. This is from the summer of 1918, when Hemingway was recovering from shrapnel wounds at the Red Cross hospital in Milan and trying to turn his wartime experiences into fiction. For the first time, he tried out the Nick Adams persona. The manuscript is at the Morgan, scrawled in pencil on Red Cross stationery.

Perhaps because of the famous “For Whom the Bell Tolls” jacket photo (also at the Morgan), which shows Hemingway bent over a Royal portable, or because of the cleanness and sparseness of his prose, we tend to think of him as someone who wrote on the typewriter. But the evidence at this exhibition suggests that, in the early days anyway, he often wrote in pencil, mostly in cheap notebooks but sometimes on whatever paper came to hand. The first draft of the short story “Soldier’s Home” is written on sheets he appears to have swiped from a telegraph office. The impression you get is of a young writer seized by inspiration and sometimes barreling ahead without an entirely clear sense of where he is going.

He began the original draft of his first novel, “The Sun Also Rises,” which he finished in just nine weeks during the summer of 1925, on loose sheets and then switched over to notebooks. It wasn’t until the end of the third notebook that he wrote a chapter outline on the back cover (which also records his travel expenses and his daily word counts, something Hemingway kept careful track of), and some of the pages on display show him slashing out not just words and sentences but whole passages as he writes. “Writing it first in pencil gives you one-third more chance to improve it,” Hemingway wrote later in an Esquire article. “That is .333, which is a damned good average for a hitter.”

F. Scott Fitzgerald (some of whose correspondence with Hemingway, beginning that year, is also on view) famously urged him to cut the first two chapters of “The Sun Also Rises,” complaining about the “elephantine facetiousness” of the beginning, and Hemingway obliged, getting rid of a clunky opening that now seems almost “meta”: “This is a story about a lady. Her name is Lady Ashley and when the story begins she is living in Paris and it is Spring. That should be a good setting for a romantic but highly moral story.” In 1929, in a nine-page penciled critique, Fitzgerald also suggested numerous revisions for “A Farewell to Arms.” Hemingway took some of these, but less graciously, and soon afterward his friendship with Fitzgerald came to an end. At the bottom of Fitzgerald’s letter he wrote: “Kiss my ass/E.H.”

The papers at the Morgan show a Hemingway who is not always sure of himself. There are running lists of stories he kept fiddling with, including one with his own evaluations: “Tour de force,” “Pretty good,” “Maybe good.” And there are lists and lists of possible titles, including the 45 he considered for “Farewell” (among the discards, thank goodness, were “Sorrow for Pleasure,” “The Carnal Education” and “Every Night and All”).

Hemingway also tried 47 different endings for that novel. Those on view at the Morgan include the so-called “Nada” ending (“That is all there is to the story. Catherine died and you will die and that is all I can promise you”) and the only slightly more hopeful one suggested by Fitzgerald, in which the world “kills the very good and very gentle and the very brave impartially. If you are none of these you can be sure it will kill you too but there will be no special hurry.”

In display case after display case, you see Hemingway during his Paris years inventing and reinventing himself, discovering as he goes along just what kind of writer he wants to be. In a moving 1925 letter to his parents, who refused to read “In Our Time,” his second story collection, he writes: “You see I’m trying in all my stories to get the feeling of the actual life across — not just to depict life — or criticize it — but to actually make it alive. So that when you have read something by me you actually experience the thing. You cant do this without putting in the bad and the ugly as well as what is beautiful.” As the years go by, he also puts on weight, grows a mustache (seen in a Man Ray photograph) and for some unfathomable reason poses for an oil painting as “Kid Balzac,” a challenger ready to knock out the great 19th-century realist.

By the time the Second World War broke out, Hemingway had solidified — fossilized even — into the iconic figure we now remember: Papa. Even J. D. Salinger calls him this, in a 1946 letter written while Salinger is in an Army psychiatric hospital, in which he says of the war that a 1944 meeting with Hemingway in Paris was “the only helpful minutes of the whole business.” Hemingway, often drinking and despondent, didn’t know it, but his best work was behind him by then, though there is perhaps an inkling of diminished expectation in a July 1949 letter he wrote to the screenwriter and novelist Peter Viertel that ends: “I don’t know any place left in the states where it’s the kind of wild I like.”

A blustery, cranky Hemingway appears in 1949 when aboard the Pilar he grabs an old fishing diary and begins scrawling an angry letter to Harold Ross, the editor of The New Yorker (whom he addresses as “Mister Harold”), complaining about Alfred Kazin’s review of “Across the River and Into the Trees,” not, in truth, a very good book. Kazim or Kasim, or whatever his name is, Hemingway tells Ross, can take his review and shove it you know where, and he will supply the grease. As Hemingway gets angrier and angrier his pencil almost goes through the paper, and then, as suddenly as it struck, the squall passes. The letter was never sent.

A more endearing writer is the one who reveals himself in a series of uncharacteristically shy wartime letters to Mary Welsh, who would become his fourth wife. In one, he apologizes for not knowing enough adjectives. In another, in a sort of stream-of-consciousness vision of intimacy apparently written in darkness while he is traveling with the infantry as a war correspondent, he says: “It would be lovely to be in bed now, legs close and all held tight and lip like when you’ve pulled the pin from a grenade and let the handle ease up under your hand.”