omnibus of: TUNNEL IN THE SKY (1955) + TIME FOR THE STARS (1956) + CITIZEN OF THE GALAXY (1957)

Tunnel in the Sky (1955) - one of Heinlein's "juveniles" from Scribners (that didn't see paperback form until 15 years after it came out.) I've read this and like it, but for a decent plot set-up, I rely on an excerpt from a review of that first paperback - by Lester Del Rey from IF January 1971:

"…It's the story of a bunch of young men and women (old enough to have children, so I don't consider them "kids") who are sent through a tube between or through or over the dimensions to another world as a test of their fitness to graduate from Survival Training. They can pick their own weapons and equipment but they're to know nothing about their destination in advance. They must survive there for two weeks before they can be returned. Those who live graduate; the others obviously have failed the course.

The going for the principals is tough enough for two weeks. But at the end of that time the pickup fails to appear. And they gradually realize that they may be stranded for years - or forever. They still know almost nothing about the dangers of the world on which they find themselves. But as best they can they have to find a way to pass the ultimate Survival Test - the survival of self and kind. Nothing is obvious from there on. But it's beautifully worked out. If you haven't read it, do so at once." [-Lester Del Rey]

Citizen of the Galaxy (1957) - serialized in Astounding, September to December 1957.

Thorby, a small, scared, dirty and defiant little boy, is sold as a slave to a one-eyed, one-legged beggar in the city of Jubblepore, Captital of the Nine Worlds. Baslim the cripple is not just any beggar - he is engaged in some sort of illegal activity involving a disguise with an artificial leg, a photo-lab in their home, and keeping track of certain starships and their activities. Thorby thinks nothing is unusual about this - "Pop" can do anything. Baslim gives Thorby an education and starts using him as runner - until the old man is killed for spying. His final request: deliver a message to the captain of the Free Trader Sisu. The Free Traders owed Colonel Baslim a great debt for rescuing a family-ship from slavery. Thorby is adopted into the Free Traders, and learns their ways, until he is transferred to the Hegemonic Guard - and then to Earth, when his true identity is finally traced. Heinlein shows us four different societies from the inside, and makes them believable and natural for those who live in them. He also gives a story-line that is impossible to put down: beggar boy makes good, then better, and finally finds a place where he "fits." One of Heinlein's best.