Another Heinlein juvenile. Our hero is the owner of a massive alien pet that's been passed down from his great(?) grandfather, one of the first interstellar travellers - although it was small enough to fit in a duffel bag when it came to Earth. Each of the humans is named "John Thomas Stuart," this one being "John Thomas Stuart XI." I don't think his age is ever stated, but I have to guess between 13 and 16. He calls his alien creature "Lummox" because its incredibly strong and has a tendency to break things. At which point he gives Lummox a stern talking to - which doesn't always work, because Lummox has a reasonable grasp on the English language and a lawyer's grasp of logic, so anything that John Thomas didn't specify as off-bounds is clearly okay ... Not that Lummox is a deliberate trouble-maker, just occasionally bored. John Thomas is, as a result, often in trouble with the local police.
This all gets more complicated when a new alien race comes calling to Earth. John Thomas isn't initially aware of this, but we see the political manoeuvrings of Secretary of Interstellar Affairs (or some such) Mr. Kiku as he attempts to satisfy their request and not have our home planet blown up. According to Wikipedia (and I suspect they're quite correct), the fact that Mr. Kiku was a black African in a position of power in an American book written in 1954 was incredibly forward-looking and novel at the time. Lummox's relation to the visiting aliens becomes important.
My favourite exchange is between an alien interpreter and one of Kiku's associates:
"All languages carry in them a portrait of their users and the idioms of every language say over and over again, 'He is a stranger and therefore a barbarian.'"
Greenberg grinned wryly. "Discouraging, isn't it?"
"Discouraging? Why, sir? It is sidesplitting. It is the only joke that God ever repeats, because its humor never grows stale."
John Thomas is an appealing character. His mother is overbearing and unpleasant, and as the book progresses, she becomes less and less believable. The political tricks and twists vary between brilliance and pure idiocy in a way that's quite astonishing: there were several points where I wondered how Heinlein could have thought something through so well, so thoroughly, and come up with such a fine solution ... and then a couple pages later he'd come up with another "solution" so sloppy and silly that even the target audience would be forced to wonder what world it could happen in.
John Thomas' romantic interest is a young woman who initially comes across as both hyper-competent and quite charming, but as the story progresses, it becomes clear that she'll ensure that everything will go her way: she's incredibly and unpleasantly domineering.
Overall, a good story that could have been great but for some rather odd missteps.