To Sail Beyond the Sunset is related to a sizeable proportion of Heinlein's science fiction. It is the fourth novel in a sequence that began with Time Enough for Love (itself a sequel of sorts to Methuselah's Children, 1941) and continued with The Number of the Beast and The Cat Who Walks Through Walls (1985). But one of the premises of these novels is the ability to travel not only through space and time, but to parallel universes, which allows Heinlein to incorporate most of the characters from his Future History series (a group of stories and novels later published as The Past Through Tomorrow, 1967), and nearly any of his other books. Characters from Methuselah's Children, The Man Who Sold the Moon (1950), The Moon is a Harsh Mistress, and Stranger in a Strange Land, all rub elbows. The effect is at times witty, but not nearly as successful...