I can't believe I'm only the second guy to recommend Bridge of Birds
By Barry Hughart. A very very funny book. I started reading it one day on a train drive (in PB) and when I got off nobody on the same car would've bet a nickel on my sanity, the way I was laughing...
The Good Soldier Švejk is a Czech book that is quite amusing - all about how stupid war and the military are.
Seconded. I think it was the inspiration for "Catch 22", another funny book.