Content warning spoilers and unwelcome frankness
I originally read this in the 80s, found a copy on the curb, and figured I'd give it a reread. This was a mistake.
For one thing, Farmer can't write Clemens. The entire conceit of the series is that you get to throw historical characters at each other; have fun with it! But no, his Clemens is dull and unfunny, terminally afflicted with Science Fiction Information Dump Syndrome. He's supposed to be leading a country, but he's just so dull you can't imagine anyone following him to the corner store for a cigar. Farmer doesn't manage to make him seem clever or even an entertaining curmudgeon.
And then we come to the other problem, which is that Farmer is of that generation of white sci-fi authors who really, desperately wanted to believe that racism was over. In the second half of the novel we're treated to pages and pages of "well, yes, racism was terrible, but now it is The Future, so please be reasonable", which ramps up into a war against a black nationalist state, rape, and eventually a stunning moment where Clemens "darkens his skin" to spy on the enemy.
It is kind of amazing that even in 1971 no one involved in editing or publishing this book sat down with Philip Jose Farmer and gently explained that he had, whether by intention or accident, written a novel in which Mark Twain wears blackface, and that he shouldn't have.
Anyway, a childhood touchstone that should never have been revisited. I tossed it in the trash.