This is one of Heinlein's "juveniles"--that is marketed towards teens, and published in 1956. So yes, it's dated in several respects, but still enjoyable. The premise is that the "Long Range Foundation" is trying to expand from an overcrowded earth to the stars, but without Faster-Than-Light (FTL) travel, communication is a challenge. Fortunately it's found that some humans, especially twin pairs are telepathic. So while Pat Barlett stays on earth, his twin Tom is on one of the starships traveling to the stars. But given relativistic physics, for every year that Tom spends on the ship, Pat is aging decades on Earth. Much of science fiction depends on positing FTL travel because it doesn't want to deal with the consequences of Einstein's theory. Those that do however often make for thought-provoking poignant stories. For instance, Joe Haldeman's Forever War. I wouldn't put this book on that level--or for that matter do I think it matches Heinlein's best books such as Starship Troopers or The Moon Is a Harsh Mistress. But worth reading if you're a science fiction fan? Absolutely!
