I love the Discworld books by Terry Pratchett. In fact, it wouldn't be exaggerating to say that the Discword series is my favorite book series ever. My opinion on this book, the 21st in the series and the 4th in the City Watch subseries, therefore, won't be much of a surprise: I loved it, devouring it in about two days flat.
Pratchett is a master of balance, creating stories that are uproariously, laugh-out loud funny, while still being able to conjure up legitimately dramatic moments. His characters are never (or very rarely) one-off jokes, and series regulars often are revealed to be incredible complex and deep characters.
Perhaps this is why the City Watch subseries (focusing on the adventures of the Ankh Morpork City Watch) is arguably the most popular amongst Discworld fans, because it has an incredibly broad and amazingly multidimensional cast, and fans can really feel like they've gotten to know characters like Captain Carrot Ironfoundersson, Sergeant Fred Colon, Corporal Nobby Nobbs, and most importantly Commander Sam Vimes, almost certainly Pratchett's most popular and well developed character.
Much of "Jingo" is told from Vimes' perspective, and this is undoubtly what makes it really work incredibly well. This is because Vimes, despite being a rather rough sort who is an expert in street combat and rather a rough bastard, sees his ultimate aim being the keeping of the peace, and his city is on the brink of literally declaring war.
A small island known as Leshp has suddenly risen from the depths of the Circle Sea, exactly equidistant between Ankh Morpork and the country of Klatch, a nation rather reminiscent of a melange of India and the Middle East. Although no one really wants Leshp (the strange buildings on the island, seemingly built for distinctly nonhuman residents, give everyone the willies), matters of national pride are bringing relations between the two countries to a rather uncomfortable boil, and things get even worse when someone attempts to assassinate a Klatchian prince in Ankh Morpork. This, combined with Ankh Morporkian nobles overly eager to earn glory in war (never mind the fact that they know almost nothing about how to wage a war) means that Vimes and his watchmen have quite a lot on their plates to deal with.
The anti-war message is very strong but is also delivered in a very interesting way. Vimes hates warfare, considering it to be in essence the ultimate crime. Trying to solve the assassination case might patch up relations, or it might just make things worse. The fact that very few people are on his side (not the Morporkian nobility, who desire glory on the backs of slaughtered soldiers, or the populace of Ankh Morpork, enflamed by racist anti-Klatchian fervor and desiring blood) makes his struggle all the more poignant.
I also loved that we are given a foil for Vimes, in the form of the mysterious Klatchian 71-Hour Ahmed, who resembles Vimes in certain ways and differs from him in one critical respect. The latter quarter of the book, which features the two essentially teamed up, really helps illuminate what kind of man Sam Vimes is.
But Vimes is not the only character to recieve the spotlight. A subplot involving a secret mission conducted by Ankh Morpork's impossibly competant ruler Lord Vetinari, and assisted by absent minded inventor Leonard of Quirm and watchmen Colon and Nobby (not exactly the cream of the City Watch crop) is full of scenes both amazing (mostly involving Vetinari) and out in out hilarious (mostly involving Nobby). Vimes' impossibly pure second in command Carrot is also given plenty of moments in which to shine, as he proves over and over his ability to inspire and bring together people who should be at each other's throats.
"Jingo" is both funny and powerful, with great characters, and interesting plot, and an unusual anti-war message. I recomend it most highly.