It is one of the 'Lazarus Long' set of books, involving time travel, parallel dimensions, free love, voluntary incest, and a concept that Heinlein named pantheistic solipsism - the theory that universes are created by the act of imagining them so that somewhere Oz is real.
Opinion is divided among science fiction fans as to whether this and other late Heinlein novels are brilliant, creative and original, or simply the wish-fulfillment of a man in his second childhood.
Other books in this set include Methuselah's Children, The Cat Who Walks Through Walls, The Number of the Beast, and Time Enough For Love.