This is distinctly a Heinlein juvenile, and it's not one of the best. It's not terrible - Heinlein is almost always readable and interesting, whether or not the story is great, or the politics worrisome. But it's not great, and certainly not as good as some of his other juvenile novels. His take on female characters is particularly weak in this one.
This is about a preternaturally intelligent young man (not uncommon in the juveniles, and at least the young female character seems to be just as intelligent, and much more savvy) has been raising his family pet, Lummox, for his whole life. So did his father. And his grandfather. All the way back to his illustrious starfaring ancestor, who brought Lummox back from one of the first successful interstellar trips.
Lummox has grown from an adorable puppy-sized creature to one much bigger than, well, just about anything. Elephant-sized, at least. Or bigger. Even though she speaks in a high-pitched adorable voice. (The gender of Lummox is never quite discussed - her species apparently has nine of them. But at the end, they refer to her as a princess a lot, and she seems to like that, so, lacking nine pronouns, I'm going with "she.")
Lummox is also hungry. For just about anything. Metals, the prize-winning roses next door, trees. Anything John Thomas hasn't specifically told her not to eat. And so, she causes quite a ruckus one day, which brings the law down on her head. The local sheriff wants her destroyed. So does John Thomas' mother, who is a two-dimensional mother caricature if I've ever seen one.
The diplomats get involved, and eventually Lummox's people coming looking for their wayward child. This is all very amusing, and the bits of powerplay amongst politicians and career bureaucrats more entertaining than John Thomas' mother yet again playing the heavy for no particular reason. I mean, she had to have accepted Lummox when she married his father, right? Then what the hell? If she has chosen to hate Lummox, give her a reason. I don't care if it's a juvenile. The character just made me sigh irritatedly at the author for such a weak villain.
So, it's a juvenile science fiction novel, and the mother is irritating, and at the end, John Thomas shows he knows how to keep his girlfriend in line after all when he forbids her from painting her face the way she wants when they get married. Yeah, there's some troubling stuff in here.
It's also very readable, because I'm not sure Heinlein knew how to write a line that wasn't readable. It's frequently entertaining, and the bureaucrats stole the novel, as far as I was concerned. I didn't really care about the young protagonists, and I think that's a weakness. The adults, when they're allowed to be adults and not just roadblocks, are far more interesting.
It's a weaker Heinlein juvenile, but still, it was mostly fun to read.