Jingo ridicules war, and that's what it centers about, but in the process it ridicules (the usual) the city of Ankh-Morpork and it's solid river, the government, the people, foreingers and anything else that Terry just happened to be thinking of. (he can't help it i guess).
It's one of the best of his books i've ever read and i've read fourteen of them so far (with no plans of stopping). I'd read a recipe of bran muffins if Terry wrote it, that's how much of an established and devoted fan i am.
The humour is very hidden at times. My advice, don't start nodding your head off anytime or you'll miss most of the jokes. This guy is a master at the art of sophisticated, witty humour. And fart jokes too (he gets to that somewhere halfway through the book; poor Nobby and Colon. Imagine stuck at a "submersive" and "marine" vehicle under the ocean, with all there is to eat is cheese and (shudder) beans. And the container you're in is sealed. Yeah, the poor crew on board the ship thought it was a sea monster).
The characters are very believable, no matter how strange they are. I can actual feel pity for Vimes, the commander of the police Watch. Then there's Captain Carrot, the dwarf who's not so dwarf-ish. I think there's so much depth in this character because this guy just happens to understand EVERYTHING about people. And it's all through pure, innocent ignorance (sounds a bit strange). Terry wonders if three is some hidden intelligence beneath all that innocent stupidity. There must be, no one is that stupid. Well, if that's the case, no one can be that patient.
Anyway, there's many layers to this book. The most important one is it's a very, very funny and entertaining book. And it's so different from anything else you've read (other than discworld).