Certainly not that well known, especially now, but one of my all time favorite sci-fi writers. Of all the sci-fi fantasy I read (which is quite a bit) I found his to be some of the most creative, original, and engrossing. He was probably best known for the Riverworld and World of Tiers series, but my favorite is the out of print Dark is the Sun. He had a good life, wrote many books that are deserved of much more attention. He will be missed by many sci-fi fans and of course his family. His official page is here.