Lumen

This famous novel, written in the form of a philosophical dialogue, features a cosmic spirit named Lumen who reveals the scientific wonders of the celestial universe to Quaerens, a young seeker of knowledge. Within its pages, the author mixes empirical observations about the nature and speed of light with vivid speculations about such diverse subjects as reincarnation, time travel, the reversibility of history and the ecospheres of alien planets. Lumen is one of the first science fiction novels to include detailed descriptions of alien life forms and the first to imagine (30 years before Einstein's theory of relativity) the differences in perception that might result from traveling at velocities close to and beyond the speed of light.