The National Endowment for the Humanities and the Ford Foundation have formed a non-profit corporation called Literary Classics of The United States to publish works by American authors that are now difficult or expensive to obtain, only sporadically available in paperback, or out of print.

The two organizations announced yesterday that they have provided $1.8 million in seed money to fund the new operation for its first four years. After that time, they hope that it can sustain itself from its book sales.

Some 15 editions already are planned for publication beginning in the spring of 1981, including works by Henry Adams, Stephen Crane, Ralph Waldo Emerson, Nathaniel Hawthorne, William Dean Howells, Washington Irving, Henry James, Thomas Jefferson, Herman Melville, Francis Parkman, Edgar Allan Poe, Harriet Beecher Stowe, Mark Twain and Walt Whitman.

Journals and travel accounts as well as history, philosophy, poetry and fiction will be presented in hardcover editions ranging from 1,200 to 1,500 pages. Each will include several works by a single author or, where feasible, the writer's complete work.

Literary critic Richard Poirier of Rutgers, vice president of Literary Classics, said that each volume will cost $15. "That's an enormously attractive price when you consider that hardcover books are often selling for about $20 these days," he said.

"It could conceivably be the most important national public project since the Federal Writers Project in the 1930s," said Harvard's Daniel Aaron, president of the new corporation. "It's a way to remind the American people of their neglected and forgotten heritage."

"The classics are too often thought of as burdens, books we were supposed to read and didn't because they seemed stuffy and difficult," said NEH chairman Joseph Duffey. "The project will demonstrate the American classics."

According to Sheryl Hurley, executive director of Literary Classics, major publishing houses will bid on distribution rights to the books some time next year.