Telepathic twins are used to keep exploration ships in contact with Earth. The fact that telepathy (in this story) is faster than light affects the physics community. Pat and his brother Tom are complex and have deep-seated emotional issues about being split up and going on a mission where the odds of the traveler returning are quite slim.
One ages, one doesn't, due to time dilation effects, and upon returning their meeting is far from simple.
In the meantime, habitable planets are discovered, Pat has to deal with teen and adult social issues aboard a crowded ship on a years long mission, most of the sister ships are lost in accidents, much of the crew of his ship die from disease or hostile aliens. He's hardly a young man himself upon return, even if his years aren't great.
And how should he feel when the faster than light aspects of telepathy lead to faster than light spaceships, leaving him a relic of a forgotten past when he does return?
Time for the Stars is action packed and scientifically consistent, with realistic and deep characters. The shipboard content is real in a way only a Naval officer like Heinlein could make it. This book and the others in his juvenile bibliography are what sucked me into science fiction, and a major contribution to me being the writer I am today. Take that as advice or as a cautionary tale, but this book is excellent even 50 years later, even after I've matured 25 years since first reading it.