I finished "Gods of Riverworld" by Philip José Farmer a while ago. I've waited a while to write this up, as I was kind of lukewarm on the last book in the Riverworld series.
In the Riverworld, people have souls ("wathans"). It doesn't matter that they were created using technology, the important thing is that a soul can take on new bodies, and continues doing so until it moves beyond its need for reality and becomes one with the "oversoul". This technological Buddishm was the central premise of the first four novels, and I liked this idea a lot.
Imagine my surprise when Farmer came along in the fifth book and tinkered with the formula. It's his to tinker with, of course. It doesn't mean I have to like it though. To me, it seems like he swapped something spiritual for something political. Instead of a state of nirvana that an individual achieves in their own way, enlightenment is now like a doctoral thesis, you get it as soon as you can convince the doctoral committee. I found this somehow less satisfying.
I have to admit that I found the book a bit tiresome for other reasons. Farmer realized that he could pull from anywhere in history, and to do this well, he obviously did an awful lot of research. After a certain point, though, we get somewhat dry backstories for each character presented as a whole. I realize it's necessary to introduce the major characters, and that their histories are relevant, but if we encounter smaller portions of their history over time, it's a bit less taxing. Iain Banks is quite good at this, bringing back hints of a larger history, showing us that his characters have a past by presenting scenes from their lives. In earlier books in the series, the main characters' previous lives were introduced over time, in segments, and as relevant to the story. By the time we get to the fifth book, Farmer is almost throwing his raw notes about each character's biography at us.
This is not to say that the book didn't have its moments. It's just not as enjoyable as the first four books, and for me at least ventures into the same kind of cringeworthy revisionism that tarnishes "Star Wars: Episode One" for anyone old enough to have seen the original series first.
Take my advice and stop with the fourth book ("The Magical Labyrinth").