Indeed, he wrote some great stuff. I loved the Riverworld series. I also adored his "
Venus on the Half Shell", written in pastiche of Kurt Vonnegut's style, and originally published under the byline of the fictional Killgore Trout. The novel reads like a precursor to the Hitchhiker's Guide to the Galaxy, and is really funny.
Anyway, Fare Thee Well. 91 years isn't a bad run on this planet. And may you wake up in a pleasant location without any grail-slavery.