Fourteen years ago, the playwright and activist Larry Kramer was preparing to die. His liver was failing, and the prognosis was grave.

He summoned an old friend, Will Schwalbe, editor in chief of Hyperion Books, and made him the steward of his novel in progress, a rambling history of homosexuality and AIDS in the United States. He told Mr. Schwalbe to self-publish it if necessary, using money from Mr. Kramer’s estate.

It didn’t come to that. Mr. Kramer survived a liver transplant and finished the book, a project he had begun more than 30 years ago. Now, at 79 and against all odds and expectations, he has lived to see it published. The first volume, “The American People, Volume 1: Search for My Heart,” weighing in at 775 pages, comes out Tuesday from Farrar, Straus and Giroux. Mr. Kramer has already thrown himself into finishing the second volume, which is to come out in 2017.

“Larry lives to write,” said Mr. Schwalbe, who is now an executive vice president at Macmillan. “There’s no doubt in my mind that Larry’s writing is what keeps him alive.”

“Magnum opus” doesn’t seem like a robust enough phrase to describe the scope of “The American People,” which stretches back to the prehistoric swamps of the Everglades and concludes, in the second volume, in contemporary New York City. When Farrar acquired the two volumes in 2010, the narrative had swelled to around 4,000 pages.

Blending farce and tragedy, autobiography and fiction, it opens as Fred Lemish, a stand-in for Mr. Kramer, is struggling to finish writing a book titled “The American People,” a far-reaching historical exposé that describes Alexander Hamilton, George Washington, Abraham Lincoln, Mark Twain and other major historical figures as gay. “He’s been struggling with this history for many years,” Mr. Kramer writes of his fictional counterpart.

Mr. Kramer said he was driven to write the book because he had long felt that gays had been excluded from history books, written out or ignored. “Most history is written by straight people, and they don’t have gaydar,” Mr. Kramer said during an interview at his apartment in Greenwich Village. “People say, ‘Can you prove to me that George Washington was gay?’ and I say, ‘Can you prove to me that he wasn’t?’ ” (As evidence, Mr. Kramer notes that Washington “was surrounded by men, and he designed all their uniforms himself.”)

In the novel, and in conversation, Mr. Kramer criticizes historians and scholars like Stacy Schiff, Ron Chernow and Doris Kearns Goodwin for glossing over homosexuality in American history. If he had had his way, he would simply have called the book a work of history rather than fiction, he said.

“Farrar Straus said call it a novel, that way the lawyers will leave you alone,” he said. “But I believe everything in the book is true. It may look like fiction, but to me, it’s not.”

Mr. Chernow, the author of a well-regarded 2004 biography of Alexander Hamilton, said he was surprised to be called out in Mr. Kramer’s novel, particularly as he brings up in his book the possibility that Hamilton might have been bisexual. “I’m glad that Larry Kramer is raising the issue, I’m just mystified at why he’s attacking me, when I thought he would have applauded the fact that I take this seriously,” Mr. Chernow said. “It’s a legitimate issue for historians or novelists, but we also have to be careful not to ransack history in service of a political agenda.” ...