Man... early Robert Heinlein really is different from middle or late Heinlein.
"Tunnel in the Sky" isn't a repeat of "The Door into Summer," but there's an unmistakable link between the two. Most fundamentally, they share a deep optimism about mankind, technology, and the future. Individuals in the book may struggle against obstacles, but in the big view, humanity is on a continuing upward trajectory.
The writing in TitS... er, let's make that "Tunnel"... is good, but perhaps just a notch worse than in TDiS. Scene-for-scene I thought it was well done, but the overall arc of the novel feels a bit off, particularly in the end... just as the book is heading for a satisfying resolution, some new plot threads are raised up in the final pages, and lets in a place where they can't get tied up. I suppose you could say that this is deliberate - showing the messiness of real life and whatnot - but after the rest of the book I don't really buy it.
One of the more remarkable aspects of the book is the language of the characters. This may be the best example I've read of the "Golly, gee-whiz, this is swell!" enthusiasm that we often associate with 1950's Americanism. Everyone talks like this, in a bright, friendly, confident yet inclusive patter. It's weird and amusing and kind of touching; again, it seems like a glimpse into an attitude that our country lost somewhere around the time of Vietnam.
So, what's this book about? Basically, it's a more cheerful and science-fiction-y version of "Lord of the Flies." It isn't all cheer; we do encounter some of the viciousness and immaturity that marred Goldman's society. Overall, though, it's a very progressive experience.
The youngsters are a bit older here, spanning late high school and early college rather than the younger crew in LotF. Interestingly, it's the oldest kids who are the most antisocial. At a couple of points during the book I felt a little like Heinlein was pandering. He may have decided that his target audience was schoolchildren, and has more than a few cases of pointing out how bright and mature they are and how adults don't appreciate them. Anyways, the students are taking place in a field survival test, and initially follow the rules of competing only individually or in two-person teams. When something goes wrong and the test fails to end, though, they must adapt to ensure long-term survival in a hostile environment. Rod, the main character, partners, then takes in a third member, then gradually grows the group to dozens of survivors.
Along the way, he experiences a series of civics lessons. With a larger group comes politics. Factions spring up, there's jockeying for power, and arguments arise over the direction of their colony/town. There are a few very subtle touches that slightly disturb the hooray-for-America tone that overlies much of the book. We see the failure of democracy, for one; a smooth-talking individual wins power in a peaceful and fair democratic election, but is a much worse leader than the competent-but-less-articulate Rod. Still, by watching how Rod responds to setbacks and continues to work for the good of the colony, our faith in the system is restored.
The underlying science of this sci-fi book is fairly interesting. It's more hand-wavy than the well-thought-out system of TDiS, but Heinlein still makes an effort to make it plausible. Portals allow mankind to instantly travel to almost any point in the galaxy; however, the portals are very expensive to operate, and end-points can only be set up after people have traveled there the old-fashioned way. (Yes, this is a system identical to the one described by Vinge. With good reason; it makes a lot of dramatic sense.) This leads to a system like that of the colony planets in Firefly, and for the same reasons. If a group of humans will need to travel for decades in space, and spend decades more establishing their settlement and building a portal, they don't want to bring along energy guns, vehicles, and other high-technology artifacts that will run out of power, break down, and be impossible to repair. Instead, people bring horses, and knives, and rope; simple equipment that allow them to scrape out an existence and, more importantly, can be repaired by unskilled people with the supplies they have on hand.
With that science as the background, Heinlein touches on several interesting social aspects, albeit without thoroughly plumbing them. First is Earth itself. There's a kind of Malthusian dynamic at work here; Earth's population has continued to grow beyond its capacity to feed itself. This is what drives much of the colonial expansion; colonies are valuable for minerals and fuel, but even more than that, they are essential to feed people back home. We see this in a few meal scenes where otherwise wealthy and privileged people must make do with a paucity of nourishment.
A second social aspect is the colonists themselves. Western society has evolved up to the point where it is highly refined and intellectual; now, it needs to sort of de-volve into a hunter-gatherer society capable of surviving in a hostile environment. Not the whole society, of course, but the most exciting opportunities in this world are those setting up new colonies, and the kind of training this requires is wholly different from the requirements of previous generations. That shift is interesting in itself, and it's also interesting to see the tensions that it creates between generations. I'm reminded of the stereotype of a father railing against his son for pursuing book-learning instead of working the land; here, the father regrets the son's attempt to return to working the land.
"Tunnel" is a rough work, but a good one. It's fun to see a master at the beginning of his career, practicing within a genre that he will later upend. It's also yet another neat bit of anthropology, catching a glimpse of a more upbeat time in American attitudes where anything seemed possible, even changing ourselves to scratch out a new existence among faraway stars.