Mark Twain (as Sam Clemens) is a major character in the Riverworld series, pointing the reader towards its most obvious precedent, Huckleberry Finn (1885), the classic account of a difficult river journey laden with symbolic overtones. In addition, Farmer has written fondly of Lord Dunsany's Tales of Three Hemispheres (1919), in which the characters travel on the boat "Bird of the River" down a fabulous legend-and-demon-haunted river. He has also drawn attention to A HouseBoat on the Styx (1895) by John Kendrick Bangs, consisting of a series of satirical dialogues between resurrected historical figures such as Shakespeare and the Emperor Nero. Bangs's book, in turn, has its roots in the tradition of the "imaginary conversation." Some critics believe that the very first science-fiction works are of this genre, which can be traced back at least to the Greek satirist Lucian, who in the second century AD wrote two volumes...