I would add the following to the list:
The Dubliners, by James Joyce (1914)
The Grapes of Wrath, by John Steinbeck (1939)
The Sun Also Rises, by Ernest Hemingway (1928)
Age of Innocence, by Edith Wharton (1921)
Light in August, by William Faulkner (1932)
Heart of Darkness, by Joseph conrad (1902)
The Metamorphosis, by Franz Kafka (1915)
My list would have plenty of classics on it; however, since the question was relatively broad, I'd like to give two unusual choices:
L. Frank Baum's Wizard of Oz and Margaret Mitchell's Gone with the Wind.
By no means do I think these are among the defining literary works of the day. They are not as representative of the human condition as, let's say The Grapes of Wrath or To Kill a Mockingbird. However, once they were adapted and made into movies, these two works became iconic classics. stories familiar to everyone.
I wouldn't assign them as summer reading in my AP literature class, but these two works deserve mention as major works of the 20th century.
Here are some of my favorite authors and pieces from the early 1900's:
*Lord Jim by Joseph Conrad
*The Wonderful Wizard of Oz by L. Frank Baum
* Kim by Rudyard Kipling
* Heart of Darkness by Joseph Conrad
*The Hound of the Baskervilles by Arthur Conan Doyle
*The Ambassadors by Henry James
*The Pit by Frank Norris
*The Call of the Wild by Jack London
*The House of Mirth by Edith Wharton (all of her works are fantastic--check out her short stories and her home-decorating books)
*The Jungle by Upton Sinclair
* White Fang by Jack London
The Poetry of Ezra Pound
The poetry of William Carlos Williams
*Howard's End by E.M. Forster
*Pygmalion by George Bernard Shaw
*Sons and Lovers by D.H.Lawrence
Poetry of Robert Frost
*The Good Soldier by Ford Madox Ford
*The Rainbow by D.H. Lawrence
*Metamorphosis by Franz Kafka
For me as a Brit you need to include some authors such as Kingsley Amis, World War Poets such as Brooke others, JG Ballard, Golding, Lively, Greene and others of their ilk. Your question though would benefit from being narrowed down - you will really only get personal opinion with it this broad.
Upton Sinclair's The Jungle, and various works by Theodore Dreiser should be included on your list. I'm not sure how early is early for you; these two authors are quite early in the 20th century. Some of the works/authors listed above are amongst my favorites, but not very early in the century.
Certainly the authors William Faulkner and John Steinbeck must be included among the best of early 20th century writers. Their short stories stand among the best ever, and several novels--such as Faulkner's Sound and the Fury and Steinbeck's Of Mice and Men and The Grapes of Wrath--are among the best of the century.
I believe that some narrowing down of the topic would be very helpful for both you and the answers you will obtain to this question. For example, American Literature has its own "major authors and works" as opposed to British or French Literature. The origins of post- colonial literature would also emerge in this time period, as the starting of the movement began to take hold. Women authors as well as authors of color also began to start the process of emergence during the early 20th Century. I think that World War I played a rather big role in helping to shape the literature of the time period, which is why I would say poets such as Yeats could be seen as major forces of creation in the first half of the 20th Century, with works such as "The Second Coming" as an example of such prominence.
This question is quite broad. A few of my favorites include: The Great Gatsby by F. Scott Fitzgerald, A Farewell to Arms by Ernest Hemingway, Call of the Wild by Jack London, Where the Red Fern Grows by Wilson Rawles, To Kill a Mockingbird by Harper Lee, and The Color Purple by Alice Walker.