I mentioned here recently that I've been re-reading Robert Heinlein's juveniles. Here's someone else who is:
Others have mentioned how seamless Heinlein's worlds are but I'm always gobsmacked by them, even now. You know that the writer was only showing the iceberg's tip of what he knew about any of his imagined outposts -- like Jubbulpore or the Free Traders' Sisu -- but what he chose to show was exactly what you needed to know. All of the needful stuff is condensed into the man's clear prose, which is never enamored with its own cleverness.
The one I'm reading now is "Starman Jones," which has a familiar Herinlein plot: Young boy yearns for space, finds a way to get there, learns much from father/mentor figures, has adventures, the end. Here's an interesting review that notes the way Heinlein folds social commentary into the novel, the sort of thing young readers miss the first time around -- at least I did.