Shortly before noon on May 6, 1884, Ulysses S. Grant entered the office of his Wall Street brokerage firm a wealthy man. Hours later, he exited a pauper.

Thanks to a pyramid scheme operated by his unscrupulous partner, Ferdinand Ward, Grant’s investment firm had instantly collapsed, wiping out his life savings. “When I went downtown this morning I thought I was worth a great deal of money, now I don’t know that I have a dollar,” the swindled Civil War hero lamented to a former West Point classmate. In fact, Grant had all of $80 to his name. His wife, Julia, had another $130. Kind-hearted strangers responded by mailing Grant checks. Desperate to pay his bills, the former U.S. president cashed them.

Still smarting from bankruptcy’s bitter sting, Grant that summer suffered from an excruciating sting in his throat as well. When he finally visited a doctor in October, Grant l_e_arned he had incurable throat and tongue cancer, likely a product of his longtime cigar-smoking habit.

Grant had been no stranger to financial misfortune. Failing as a farmer and a rent collector prior to the Civil War, he lived in a log cabin that he dubbed “Hardscrabble” and sold firewood on the streets of St. Louis to make ends meet. However, now that he was confronting the terrifying prospect of leaving Julia a penniless widow, the grizzled general who fought to save the Union undertook one final mission to save his family from impoverishment.

Divested of his property and possessions, Grant still retained something of great value—his recollections of past glories. Lurking behind the taciturn façade was a convivial storyteller who entertained friends such as Mark Twain with yarns of war and politics. “While we think of Grant as silent and reserved, he was a captivating raconteur with a dry wit and a ready fund of stories,” says Ron Chernow, Pulitzer Prize-winning author of Grant__.

For years Twain had suggested that Grant pen his memoirs. Now destitute, the former president finally agreed to cash in on his celebrity. In need of financial rescue himself after a series of failed investments, the debt-ridden Twain inked Grant to a contract with his newly launched publishing house and gave him a $1,000 check to cover living expenses.

Engaged in a furious race against time as the cancer attacked his body, Grant dug into his writing with military efficiency, churning out as many as 10,000 words in a single day. “Grant approached his memoirs with the same grit and determination as he tackled his Civil War battles,” says Chernow, who also serves as executive producer of HISTORY’s documentary series “Grant.” “As in those encounters, he was thorough and systematic, a real stickler for precision and the truth. In his home, he amassed tall stacks of orders and maps that helped him to recreate his most famous battles with minute fidelity. In war and in writing, Grant had the most amazing ability to marshal all his energy in the pursuit of a single goal.”

Grant astounded Twain with not just the quantity, but the quality of his prose. “Grant prided himself on his writing skills,” Chernow says. “His wartime orders were renowned for their economy and exactness, and he made a point of writing all his own speeches as president—something unthinkable today.”

Grant penned his manuscript until his hand grew too feeble in the spring of 1885, forcing him to employ a stenographer. Even speaking, however, became laborious as his condition deteriorated. Following the advice of doctors who vouched for the salubrious power of pure mountain air, Grant decamped at the onset of summer from his Manhattan brownstone to an Adirondack resort north of Saratoga Springs. In a cottage on the slopes of Mount McGregor, Grant launched his final campaign to complete his tome.

With excruciating pain accompanying every swallow, Grant was unable to eat solid food. His body withered by the day. The voice that once commanded armies could barely muster a whisper. “He endured great pain with incredible stoicism,” says Ben Kemp, operations manager at the U.S. Grant Cottage State Historic Site. While Grant’s doctors gave him morphine only sparingly in order to keep his mind clear for writing, they swabbed his throat with cocaine to provide topical pain relief and used hypodermic needles to inject him with brandy during the worst of his coughing fits.

Through it all, Grant persisted in honing his manuscript—editing, adding new pages and poring over proofs of his first volume—as he sat on the cottage porch on even the steamiest of days swaddled in blankets, a wool hat and a scarf covering his neck tumor, which was now “as big as a man’s two fists put together” according to the New York Sun. When his voice finally abandoned him, Grant scribbled his thoughts in pencil on small slips of paper.

When Twain visited Grant at the cottage, he brought the good news that he had already pre-sold 100,000 copies of the autobiography. A relieved Grant knew he had succeeded in giving Julia and his children financial security. “Taking care of his family is all that mattered at that point,” Kemp says. “Grant knew at that moment this was going to be a success. Like in a battle, that was the moment he knew the tide had turned.”

With his mission accomplished, Grant finally laid down his pen on July 16 after crafting a herculean 366,000 words in less than a year. “There is nothing more I should do to it now, and therefore I am not likely to be more ready to go than at this moment,” he wrote. Seven days later, Grant’s pulse flickered and ultimately gave out.

Employing an army of door-to-door salesmen, Twain sold more than 300,000 copies of the Personal Memoirs of Ulysses S. Grant. The two-volume boxed set even outsold Twain’s latest work, Adventures of Huckleberry Finn, and resulted in Julia Grant receiving $450,000 in royalties (equivalent to $12 million today).

Grant’s memoir proved not just a commercial success, but a literary one as well. Although Grant omitted discussion of his presidency or sensitive personal matters such as his drinking, many scholars consider his autobiography the finest memoir ever penned by an American president and perhaps the foremost military memoir in the English language. “The emotion of the situation probably lent energy and eloquence to his work,” Chernow says. “In all likelihood, he had narrated many of these stories in the years since the war and they had acquired a certain smoothness and polish in the retelling.”

“There was no doubt among his family and friends that Grant had willed himself to stay alive to complete the book,” Chernow says. “He may have originally undertaken the memoirs to provide for his wife after his death, but it must also have soothed and consoled him at the end of his life to recount his glorious victories in the Civil War.”