A House-Boat on the Styx

The premise of the book is that everyone who has ever died (up until the time in which the book is set, which seems to be about the time of its publication) has gone to Styx. This does not appear to be the conventional Hell described by Dante in The Inferno, but rather the Hades described in Greek myth (both of which had Styxes): a universal collecting pot for dead souls, regardless of their deeds in life.