James Blish 
Thing in the Attic [PDF ebook] 

If you’re looking for a fun, fast, and fascinating science-fiction read, try "The Thing in the Attic" from author James Blish. This tale imagines a universe populated by tree-dwelling beings who regard life on the flat surface of the planet to be the worst possible punishment. When a small group of them are relegated to life on the surface as penance for the crime of expressing skepticism about the sacred text that governs their culture, they’re thrust into a strange new world in which they must fight for their survival.