Anne recommended to me a book called The Mouse That Roared by Leonard Wibberley. It's an older book, and appears to be somewhat obscure -- it was published in the 1950s, and the edition I have is a reprint from 1979 from a small-press that says it's limited to 100 copies, so it can't be a huge seller.
The book centers around a small European nation, the Duchy of Grand Fenwick, which has maintained its 14th-century standard of living into modern times, but finds itself running short on resources. After some discussion, they decide to declare war on the United States so that, when they lose, they might benefit from some Marshall Plan-style largesse. Only, things don't quite go according to plan, and the war is rather more successful than anyone expected.
It's an odd book, veering between farcical humor and earnest political philosophy (particularly regarding the role that small nations play in world affairs, and the dangers of the "Q-bomb", a weapon far more powerful than a mere atomic bomb); it's a bit hard to tell what to make of it, actually. I'm not certain if it's a polemic dressed up in humorous guise, or a humorous novel with a more serious point. In either event, those two elements of the book aren't especially well-integrated.
Still, though, it's a good book. It's enjoyable to read, there are a few genuine laugh-out-loud moments, and the plot breezes along. I'd probably stop short of saying that it's a great book, but if you happen to stumble on it, it's worth a read.