I adored the World of Tiers books when I was a teen-haven’t read them in awhile (loved #1, liked #3 a lot, the rest were a cut below in quality), perhaps I’ll make some time tho, for old times’ sake. Riverworld was a great premise run into the ground. Still he could tell a rip-roaring adventure like few others could.
He also introduced me to Doc Savage with his “biographical” Doc Savage: His Apocalyptic Life.
Riverworld has got to be the greatest concept in SF history - the possibilities for plots and characters are endless. I understand the usual atrocity was committed when it was done as a movie several years ago. But if it was done well, the series could live forever.
Damnit.
See you on the river, Mr. Frigate. 
I had the opportunity to ask Vonnegut about his relationship with PJF in 1997. Vonnegut was actually OK with Farmer’s doing the book - it’s when Farmer wanted to extend the Trout name into additional stuff (movies, other books, IIRC) that Vonnegut said “enough”. KV also had never read VOTHS, although he stated that “he’d heard it was actually a pretty good book”. The feud was a shame, in my book anyway.