The University of Adelaide Library has made the books of influential science fiction writer/philosopher Olaf Stapledon freely available from their website, and in a number of formats (ePub, Kindle, web-view). There's some wonderful, foundational themes in Stapledon's writing, and plenty of the type of deep thinking we like to do here at TDG:
Stapledon's writings directly influenced Arthur C. Clarke, Brian Aldiss, Stanislaw Lem, C. S. Lewis and John Maynard Smith and indirectly influenced many others, contributing many ideas to the world of science fiction. The "supermind" composed of many individual consciousnesses forms a recurring theme in his work. Star Maker contains the first known description of what are now called Dyson spheres. Freeman Dyson credits the novel with giving him the idea. Last and First Men features early descriptions of genetic engineering and terraforming. Sirius describes a dog whose intelligence is increased to the level of a human being's.
Stapledon's fiction often presents the strivings of some intelligence that is beaten down by an indifferent universe and its inhabitants who, through no fault of their own, fail to comprehend its lofty yearnings. It is filled with protagonists who are tormented by the conflict between their "higher" and "lower" impulses.