In oft-praised but rarely-read writers, I would suggest Olaf Stapledon. In Star Maker, in 1937, he even invented the idea of a Dyson sphere. Somewhat like C.S. Lewis, he approached his fiction from a philosophical perspective, but unlike Lewis, was not a Christian.
Another author nearly entirely unread these days is Auguste de Villiers de l'Isle-Adam. His novels Claire Lenoire and L'Ève future both have science fictional elements (Eve is about an android), with the latter being overtly SF well ahead of its time of publication in 1880. He approached his fiction from the school of Symbolism. Additionally, the very term "android" itself, while existing since the early 18th Century, was popularized by this novel.
Probably his most well known book is his collection of short stories, Contes cruels, in which most stories end with a twist and is somewhat reminiscent of Guy de Maupassant.
And my final suggestion, from a book I had never met anyone who has read it since the person who introduced it to me, is A Voyage to Arcturus by David Lindsay, published in 1920. All I can say about it is it literally is unlike anything else anyone ever wrote.
