If you haven't devoured the whole Dortmunder series by now, wait no longer: start with this one, and as soon as your stomach muscles recover, move on to the rest. This is the kickoff to the funniest mystery series ever written.
Our hero is Dortmunder, a very recent ex-con. His opponent is the Balabomo Emerald, a gem with a history, a price on its head, and, apparently, a nasty sense of humor. Hired by the representative of an African nation that wants the jewel for religious and political reasons, Dortmunder assembles a team of the finest, funniest men ever to make a dishonest dollar. They make attempt after attempt (as the blurb says, giving new meaning to the term repeat offender) at the job, striking by helicopter, train, and mesmerism, among others. Any one of the plans should succeed. But fate - or the emerald - has other ideas.
The plot is only part of the humor, though. In a way, the Dortmunder series is very like Terry Pratchett's Discworld series; they both rely on language and character as much as situations for laughs, and they both get incredible results from the strategy.
Seriously, this book (and the series it spawned) is a better antidepressant than any pharmaceutical I've ever encountered. If you're just now starting here, wow, do I envy you. (But how did you live this long without Dortmunder?)