In this novel, we begin with the understanding that telepathy exists, primarily in twins, but also in other close relatives, and that it can be cultivated among close relatives. Our narrator, a twin who seems to resent his brother’s more or less overbearing and dominant nature, is telling us about this ability, and then how it led to the two brothers to be recruited for space colonization. Using the telepathic powers as a kind of ancillable system, one twin would stay on Earth, while the other explores the galaxy. The novel then is mostly about jumping from planet to planet, looking for possible new colony spaces, while also dealing with the huge jumps in time on Earth, and short spans of time in local, relative time aboard the ship. It’s also got plenty of twins griping at each other.
I never quite know if I am going to really enjoy a Robert Heinlein novel going in or not. I also never quite know where the novel is going to break on the weirdness scale. Similar to other writers that he influenced, like say Philip K Dick, his novels often circulate on a conceit, but then take it to some truly strange places. The Sixth Column took a futuristic guerilla warfare novel and ended up creating a new religion. A similar blasting off from the original concept happens in The Puppet Masters.
This book seemed to influence, or contribute to the influence, in part of Ursula Le Guin, Orson Scott Card, and even the book Singularity by William Sleator, where a twin uses a black hole to age himself one year older than his brother.