Farmer's ""Riverworld"" series embodies one of the most original ideas in current sf: the resurrection of nearly the entire human race--cavemen to astronauts--on a world apparently consisting of nothing but a single river valley about 15 million miles long. Like its predecessors, this fourth and concluding volume merrily entangles the fates of Sam Clemens, Richard Burton the explorer, Lewis Carroll's original Alice, Cyrano de Bergerac, a repentant Hermann Goering, an sf writer named Peter Jairus Frigate, and anybody else that Farmer (who also provides the fictional PJF with a mysterious double) can squeeze in. Unfortunately the squeezing process involves giving everybody something to do, and here the series has become mired down in a lot of argle-bargle about the shadowy and perhaps sinister custodians or makers of the Riverworld. Clemens, Burton, and others have been pursuing the mystery long enough to have built a whole edifice of theory and jargon that--like the rivalry between Clemens and King John--merely clutters up the story of the adventurers' repeated attempts to fly, sail, or crawl to the source of things at the near-inaccessible north pole. For what it's worth, Burton and Alice do get there this time, are let in on the extraterrestrial plans that produced the Riverworld, and confront the threatened destruction of the whole scheme. But Farmer's writing is as careless as ever, and the sf revelations are infinitely duller than the unknowns with which the series began.