He began writing various essays as early as 1908, but it was not until 1930 that he published his first novel, Last and First Men. This novel, along with almost all of his other major works, is definitely SF, but Stapledon can hardly be considered a genre writer. Relatively unknown to genre readers, he nevertheless can be seen to have influenced many SF writers. I'm no scholar, but I can see his influence on Asimov, Clarke, Heinlein, Simak, Blish, Aldiss, Cordwainer Smith, and probably many others with which I'm not as familiar.

"OS is...sometimes dimly perceived [to be] the Star Maker behind many subsequent stories of the far future and galactic empires...his influence, both direct and indirect, on the development of many concepts which now permeate genre sf is probably second only to H. G. Wells." - [Mark Adlard, from The Encyclopedia of Science Fiction]

Last and First Men, along with 1937's Star Maker, have to be viewed as examples of the most ambitious fiction ever attempted. The former employs a timescale of nearly two billion years in its description of the history of mankind through eighteen distinct stages of evolution on Earth and other planets. The story is told by one of the Last Men (of the 18th stage), working through the mind of a "docile but scarcely adequate brain" of one of the First Men (ourselves). Stapledon implies he is that First Man, compelled to write about what the Last Men reveal to him. Two years later he followed up on this concept with Last Men in London, which features a character and events that closely mirror things Stapledon experienced during WW1. Several concepts introduced would later prove to be staples [pun intended] of many genre works, including genetic engineering and the terraforming of other planets to make them suitable for human habitation. Hailed by critics and other contemporary writers on its initial release, Last and First Men would later be nearly forgotten until the late '40s, when much to his surprise, Stapledon was embraced by the SF community.

EDIT, December 12, 2021: The almost unthinkable has been done. Last and First Men has been adapted into a surreal film by Jóhann Jóhannsson.

Star Maker can in some ways be considered a sequel to Last and First Men (or vice-versa), in that it takes the perspective of history away from the narrow scope of humanity and focuses it on the grander scale of the entire cosmos. The tale is related by an English gentleman who inexplicably begins an astral (out-of-body) journey which takes him high above the Earth's surface. At first fearful of his plight, he slowly becomes aware that he can will his movement through space by mere thought. After an immeasurable amount of time he encounters another spirit such as himself, and they discover they can combine the powers of their minds to travel faster through the infinities of space. Eventually they are joined by countless others as they observe many forms of civilizations on other planets in far-flung solar systems, some quite human-like, others so alien as to defy comprehension. Each of the travelers must also face the fact of their own race's pitifulness when they encounter the Star Maker itself, "the supreme moment of the cosmos."

Several themes in these novels would preoccupy most of his other work, both fiction and non-fiction, particularly the concept of community as a necessity for individual achievement, and a general acknowledgement of the inadequacy of the human mind for the discovery of truth. His other notable SF work includes: Odd John (1935), concerning a race of supermen, whose superiority is more of a spiritual and intellectual nature rather than one of "super-powers" familiar from comics and other fantastic tales; Sirius (1944), a tale of a genetically-enhanced dog with intellectual prowess, considered by many to be the best SF tale with a non-human protagonist; the novella "The Flames" (1947), which relates the plight of alien beings, once inhabitants of the sun, whose essence can be released on Earth by the super-heating of igneous rock. Death Into Life (1946), while not science fiction by strict definition, has to be considered speculative fiction of the highest order. It explores the after-life experienced by the crew of a World War 2 bomber, all killed instantly when their plane explodes in mid-air. If it wasn't already obvious, Stapledon's fiction is as far from the mainstream as possible, maybe even futher away from the type of SF you usually read. They are more like philosophical treatises than narrative novels. If that sounds uninteresting to you, look elsewhere. I'll still be recommending him years from now.

It is possible the majority of Stapledon's work is in the public domain, since there are many various editions, both print and e-book, and omnibuses that combine several titles. Some are even identified as from "Create Space Independent Publishing." Maybe not public domain, since none are available at Project Gutenberg, perhaps just insufficient sales to keep them in print. Here are some links that seem to be in-print, or at least editions that are available from various warehouses. Some of the Bookshop links say Back-Ordered, which might actually mean not available. A purchase through our links may earn us a commission.

Last and First Men - Bookshop / Amazon
Last Men in London - Bookshop / Amazon
Star Maker - Bookshop / Amazon
Last and First Men + Star Maker - Bookshop / Amazon
Odd John + Sirius - Bookshop / Amazon
Odd John - Bookshop / Amazon
Sirius - for Kindle
Death Into Life - for Kindle
The Flames - for Kindle, also available in the print collection An Olaf Stapledon Reader - Bookshop / Amazon