Updated: thanks to everyone for their suggestions and additions. The list is now significantly longer than it was when I first started putting it together.
A Note on Method: This list is cobbled together from magazine articles related to Public Domain Day, Wikipedia lists of books published in 1923, and Balfour Smith's extensive spreadsheet of works.
Works published in 1923 are now out of copyright (the reasons for this are complicated; look up the Sonny Bono Copyright Extension Act for more, or see this article in Smithsonian Magazine for a quick primer). I expected there would be a big rush of digitization to coincide with "Public Domain Day," but thus far there doesn't appear to have been all that much activity.
Perhaps one reason is that many texts now entering the public domain can already be viewed online in page view / PDF at Hathi Trust (see links below). A couple of Edgar Rice Burroughs novels show up at Project Gutenberg Australia. But the number of working plain text or HTML editions at sites hosted in the U.S. is quite small. Moreover, a number of major texts appear to have no digital versions available at all at present (see especially e.e. cummings' Tulips and Chimneys and Wallace Stevens' Harmonium).
It seems worth mentioning that a lot happened in 1923. The British authorities seized a copy of Ulysses in the mail and declared it obscene in January. The pulp magazine Weird Tales published its first issues. Sean O'Casey's The Shadow of a Gunman had its debut, as did George Bernard Shaw's Saint Joan. The Surrealists and the Dadaists had a riot in Paris, and decided to part ways. And all of the books below were published!
Here's a longish list of texts that were published in 1923, and that are now out of copyright. Where I've been able to find Hathi Trust, Gutenberg, or Archive.org links I've provided those. I will add to this list as I learn of more.
Works Published in 1923 -- Now in the Public Domain
Poetry
Jean Toomer, Cane (Hathi Trust). UPDATE: My bare-bones digital edition here.
e.e. cummings, Tulips and Chimneys (no edition as of yet)
Joseph Conrad, The Rover (Hathi Trust)
Robert Frost, New Hampshire (Gutenberg Edition: January 4, 2019)
Robert Frost, Selected Poems
William Carlos Williams, Spring and All (Wikipedia)
Aldous Huxley, Antic Hay (Hathi Trust)
D.H. Lawrence, Kangaroo (Hathi Trust)
D.H. Lawrence, Three Novellas (The Fox, The Captain's Doll, The Ladybird)
D.H. Lawrence, Birds, Beasts and Flowers
Ernest Hemingway, Three Stories and Ten Poems (Wikipedia)
Katherine Mansfield, The Doves' Nest and Other Stories (Hathi Trust)
Katherine Mansfield, Bliss and Other Stories
Edith Wharton, A Son at the Front (Hathi Trust)
H.G. Wells, Men Like Gods (Hathi Trust; Wikipedia)
Willa Cather, A Lost Lady (Hathi Trust)
William Carlos Williams, The Great American Novel
Samuel Hopkins Adams (publishing as Warner Fabian), Flaming Youth (Wikipedia Entry. Hathi Trust lists this as published 1924)
Sherwood Anderson, Many Marriages (Hathi Trust; Wikipedia entry)
Sukumar Ray, Abol Tabol (Wikipedia)
Dhan Gopal Mukerji, Jungle Beasts and Men
John Dos Passos, Streets of Night
Carl Van Vechten, The Blind Bow-Boy
Djuna Barnes, A Book
Gertrude Atherton, Black Oxen (Gutenberg link.)
Arnold Bennett, Riceyman Steps (class study involving shell shock; Wikipedia)
Translations
Anton Chekhov, Love and Other Stories (trans. Constance Garnett)
Jules Verne, The Castaways of the Flag (first English-language edition)
Jules Verne, The Lighthouse at the End of the World (first English-language edition)
Notable Nonfiction
Dhan Gopal Mukerji, Caste and Outcast
Bertrand Russell, The Prospects of Industrial Civilization
G.K. Chesterton, Fancies Versus Fads
Winston Churchill, The World Crisis
Jessie Conrad, A Handbook of Cookery for a Small House
Arthur Conan Doyle, Our American Adventure
Theodore Dreiser, The Color of a Great City
E. M. Forster, Pharos and Pharillon
James G. Frazer, Folk-lore and the Old Testament (abridged edition)
Aldous Huxley, On the Margin: Notes and Essays
D.H. Lawrence, Studies in Classic American Literature
David Lloyd George, Where Are We Going?
H.L. Mencken, The American Language, 3rd revised edition
Thorstein Veblen, Absentee Ownership and Business Enterprise in Recent Times: The Case of America
Woodrow Wilson, The Road Away from Revolution
Popular fiction and Genre Fiction
Edgar Rice Burroughs, Tarzan and the Golden Lion (Gutenberg Australia)
Edgar Rice Burroughs, The Girl From Hollywood (Gutenberg Australia; Wikipedia Entry)
Agatha Christie, The Murder on the Links
Kahlil Gibran, The Prophet (Gutenberg)
James Branch Cabell, The High Place: a Comedy of Disenchantment (Wikipedia entry)
Jeffery Farnol, Sir John Dering
J.S. Fletcher, The Charing Cross Mystery (Gutenberg Canada)
Zona Gale, Faint Perfume
Garet Garrett, Cinder Buggy
Philip Gibbs, The Middle of the Road
Talbot Mundy, The Nine Unknown (orientalist fantasy involving the Emperor Ashoka and Kali worshippers)
May Sinclair, Uncanny Stories (illustrated by Jean de Bosschere)
James Stephens, Deirdre
Margaret Wilson, The Able McLaughlins (Wikipedia)
A Note on Method: This list is cobbled together from magazine articles related to Public Domain Day, Wikipedia lists of books published in 1923, and Balfour Smith's extensive spreadsheet of works.
Works published in 1923 are now out of copyright (the reasons for this are complicated; look up the Sonny Bono Copyright Extension Act for more, or see this article in Smithsonian Magazine for a quick primer). I expected there would be a big rush of digitization to coincide with "Public Domain Day," but thus far there doesn't appear to have been all that much activity.
Perhaps one reason is that many texts now entering the public domain can already be viewed online in page view / PDF at Hathi Trust (see links below). A couple of Edgar Rice Burroughs novels show up at Project Gutenberg Australia. But the number of working plain text or HTML editions at sites hosted in the U.S. is quite small. Moreover, a number of major texts appear to have no digital versions available at all at present (see especially e.e. cummings' Tulips and Chimneys and Wallace Stevens' Harmonium).
It seems worth mentioning that a lot happened in 1923. The British authorities seized a copy of Ulysses in the mail and declared it obscene in January. The pulp magazine Weird Tales published its first issues. Sean O'Casey's The Shadow of a Gunman had its debut, as did George Bernard Shaw's Saint Joan. The Surrealists and the Dadaists had a riot in Paris, and decided to part ways. And all of the books below were published!
Here's a longish list of texts that were published in 1923, and that are now out of copyright. Where I've been able to find Hathi Trust, Gutenberg, or Archive.org links I've provided those. I will add to this list as I learn of more.
Works Published in 1923 -- Now in the Public Domain
Poetry
Jean Toomer, Cane (Hathi Trust). UPDATE: My bare-bones digital edition here.
e.e. cummings, Tulips and Chimneys (no edition as of yet)
Joseph Conrad, The Rover (Hathi Trust)
Robert Frost, New Hampshire (Gutenberg Edition: January 4, 2019)
Robert Frost, Selected Poems
William Carlos Williams, Spring and All (Wikipedia)
Wallace Stevens, Harmonium (Wikipedia)
Carl Sandburg, Rootabaga Pigeons (Hathi Trust)
Willa Cather, April Twilights and Other Poems
Willa Cather, April Twilights and Other Poems
Vachel Lindsay, Collected Poems
Vachel Lindsay, Going-to-the-Sun
Edna St. Vincent Millay, The Harp-Weaver, and Other Poems
George Santayana, Poems, revised
Fiction (literary)
Aldous Huxley, Antic Hay (Hathi Trust)
D.H. Lawrence, Kangaroo (Hathi Trust)
D.H. Lawrence, Three Novellas (The Fox, The Captain's Doll, The Ladybird)
D.H. Lawrence, Birds, Beasts and Flowers
Ernest Hemingway, Three Stories and Ten Poems (Wikipedia)
Katherine Mansfield, The Doves' Nest and Other Stories (Hathi Trust)
Katherine Mansfield, Bliss and Other Stories
Edith Wharton, A Son at the Front (Hathi Trust)
H.G. Wells, Men Like Gods (Hathi Trust; Wikipedia)
Willa Cather, A Lost Lady (Hathi Trust)
William Carlos Williams, The Great American Novel
Samuel Hopkins Adams (publishing as Warner Fabian), Flaming Youth (Wikipedia Entry. Hathi Trust lists this as published 1924)
Sherwood Anderson, Many Marriages (Hathi Trust; Wikipedia entry)
Sukumar Ray, Abol Tabol (Wikipedia)
Dhan Gopal Mukerji, Jungle Beasts and Men
John Dos Passos, Streets of Night
Carl Van Vechten, The Blind Bow-Boy
Djuna Barnes, A Book
Gertrude Atherton, Black Oxen (Gutenberg link.)
Arnold Bennett, Riceyman Steps (class study involving shell shock; Wikipedia)
Elizabeth Bowen, Encounters (Archive.org; short stories)
John Galsworthy, Captures
John Galsworthy The Burning Spear
Rudyard Kipling, Land and Sea Tales for Boys and Girls
Vita Sackville-West, Grey Wethers
Olive Schreiner, Stories, Dreams and Allegories
Virginia Woolf, "Mrs. Dalloway in Bond Street" (short story that would later feed into Mrs. Dalloway [1925])
Anton Chekhov, Love and Other Stories (trans. Constance Garnett)
Jules Verne, The Castaways of the Flag (first English-language edition)
Jules Verne, The Lighthouse at the End of the World (first English-language edition)
Colette, Green Wheat
Alexandre Dumas, The Three Musketeers (trans. Philip Shuyler Allen)
Nikolai Gogol, Dead Souls (trans. Constance Garnett)
Nikolai Gogol, The Overcoat and Other Stories (trans. Constance Garnett)
Maxim Gorky, My University Days (trans. Louis P. Lochner)
Knut Hamsen, Victoria (trans. Arthur G. Chater)
Heinrich Heine, Poems (trans. Louis Untermeyer)
Emond Rostand, Cyrano de Bergerac (trans. Brian Hooker)
Dhan Gopal Mukerji, Caste and Outcast
Bertrand Russell, The Prospects of Industrial Civilization
G.K. Chesterton, Fancies Versus Fads
Winston Churchill, The World Crisis
Jessie Conrad, A Handbook of Cookery for a Small House
Arthur Conan Doyle, Our American Adventure
Theodore Dreiser, The Color of a Great City
E. M. Forster, Pharos and Pharillon
James G. Frazer, Folk-lore and the Old Testament (abridged edition)
Aldous Huxley, On the Margin: Notes and Essays
D.H. Lawrence, Studies in Classic American Literature
David Lloyd George, Where Are We Going?
H.L. Mencken, The American Language, 3rd revised edition
Thorstein Veblen, Absentee Ownership and Business Enterprise in Recent Times: The Case of America
Woodrow Wilson, The Road Away from Revolution
Popular fiction and Genre Fiction
L. Frank Baum, The Cowardly Lion of Oz
Edgar Rice Burroughs, The Girl From Hollywood (Gutenberg Australia; Wikipedia Entry)
Agatha Christie, The Murder on the Links
Marie Corelli, Love and the Philosopher
Austin Hall, People of the Comet (Science fiction serialized in Weird Tales in 1923)Kahlil Gibran, The Prophet (Gutenberg)
Herman Hesse, Demian (first English-language edition)
P.G. Wodehouse, Mostly Sally
Maxwell Bodenheim, Blackguard
Thomas Alexander Boyd, Through the Wheat (an American World War I novel; Wikipedia entry)
Dorothy Sayers, Whose Body? (Wikipedia)
P.G. Wodehouse, The Inimitable Jeeves (Archive.org)
P.G. Wodehouse, Leave it to Psmith (Hathi Trust)P.G. Wodehouse, Mostly Sally
Maxwell Bodenheim, Blackguard
Max Brand, Seven Trails
John Buchan, Midwinter (Gutenberg Australia)James Branch Cabell, The High Place: a Comedy of Disenchantment (Wikipedia entry)
Hall Caine, The Woman of Knockaloe
Susan Ertz, Madame ClaireJeffery Farnol, Sir John Dering
J.S. Fletcher, The Charing Cross Mystery (Gutenberg Canada)
Zona Gale, Faint Perfume
Garet Garrett, Cinder Buggy
Philip Gibbs, The Middle of the Road
Talbot Mundy, The Nine Unknown (orientalist fantasy involving the Emperor Ashoka and Kali worshippers)
Liam O'Flaherty, Thy Neighbour's Wife
Olive Higgins Prouty, Stella Dallas (Wikipedia entry)
William MacLeod Raine Iron Heart
Rafael Sabatini, Fortune's FoolMay Sinclair, Uncanny Stories (illustrated by Jean de Bosschere)
James Stephens, Deirdre
Margaret Wilson, The Able McLaughlins (Wikipedia)
Anzia Yesierska, Salome of the Tenements