Time For the Stars was first published in 1956. It was one of Heinlein’s Juveniles — a series of books he wrote in the fifties with young heroes in the near future. The book is slightly dated — less so than some of the others that have more noticeable computers in them — but not really all that much. The story is an exploration of the Twin Paradox — a thought experiment that explains how relativity works. If you had identical twins, and one of them accelerated away from Earth and the other stayed home, so much more time would pass on Earth than in the spaceship that the Earth twin would be a hundred years old when the space twin came home, only a few years later. Heinlein took this concept and made it a real story with characters — and he made the twin thing relevant by using twin telepathy (which works faster than light…) as a means of communicating between Earth and ship.