The book I think is most underrated: David Lindsey, A Voyage To Arcturus. Philip Pullman
David Lindsay (1876-1945) was a Scottish novelist. His initial career, as an insurance clerk in London, was interrupted by his service in World War One. After the war, Lindsay moved to Cornwall to pursue writing. His first novel, A Voyage to Arcturus, sold poorly but over time became a cult favourite and had a profound influence on generations of writers and critics, including C.S. Lewis, J.R.R. Tolkien, Harold Bloom, and Philip Pullman. It is now considered a masterpiece of metaphysical science-fiction, and David Lindsay a pioneer of the form.