Ok I have to confess I am not an expert on Farmer's body of work. But I did enjoy the first two books of the Riverworld series and he is on my summer reading list. So I expect to read the others in the series as well. Aside from that let me say that if you get the chance and your deck is clear, read both To Your Scattered Bodies Go and the Fabulous Riverboat. These two books are the first in Farmer's Famous Riverworld Saga.
The premise of this story is that people who have died have ended up on this world that is one long continious river. They have all been cured of whatever they were dealing with on Earth and are outfitted with a small cup that they can put on these rock formations and they get food and whatever other sustinence they need. Add to that they are naked and hairless when they first get there.
The main characters are Sir Richard Burton the explorer and Mark Twain. Mark Twain in both stories builds a riverboat with the intent to explore. Both characters, sometimes together, battle people who wish to exploit them and they have harrowing adventures.
I read the Riverworld series when I was college and wonder if it still holds up to my 43 year old scrutiny. We shall see.