For the Terry Pratchett fans out there, nothing more need be said. It's Pratchett, you want to read it, the only reason you've been hesitating is because it's marked as a kids book (juvenile, young adult...) But this one isn't just for kids. As with any Pratchett book, there are layers and layers, and some of them wouldn't be obvious to kids at all.
In fact, some of them wouldn't be obvious to adults who haven't taken a college physics course or two, and/or kept up by reading all the science magazines. I'll bet I missed a couple of jokes or two, maybe a pun here and there, because my college physics courses were too long ago.
But that's OK, the book's enjoyable even without those - there are enough layers that there is something for everyone. The humor flows from the characters, the story, and the writing style. As with any Pratchett book, the humor also contains some serious ideas, hidden until you suddenly realize you need to pay attention to them.
The protagonists are Johnny, and his friends Wobbler (who wobbles), Bigmac (who is large), and Yo-less, who is apparently the only black in Blackbury who doesn't say yo. Each of this team has his own strange store of skills or knowledge, and Johnny's erratic talents turn out to include being able to talk to the dead, who definitely don't like being referred to as ghosts. The dead are characters too, especially Mr. Einstein - not the famous one, but his distant cousin, who should have been famous too, but was too busy being a butcher.
As you're reading, take note of the project on World War II that Johnny is doing for school; it also features in the next book in the series, "Johnny and the Bomb." (One of the funny bits in the book is how, whenever a kid claims he's doing "a project," he winds up with all sorts of information that is unsuitable for kids, and/or hitherto classified or secret; the remembered horror of school projects makes all the adults give in so that they don't have to think about it any more!
The series has no noticeable sexual content, and no real bad language; the most dangerous things in it for young readers are the ideas, which may make them *gasp* think! It may also make them lifelong Pratchett addicts. Some parents may object to the casual car thievery that is one of Bigmac's habits, and perhaps might find that the ability of kids to evade adult supervision is disconcerting, but it's no less true for all that. And some American younger readers may find the Englishness of it a bit hard to follow in some spots - housing projects referred to as blocks, french fries as chips, and so on. Well, a little bit of education in the ways of the world outside the USA won't hurt them any!
