It's hard to say what is the funniest book I've ever read, because humour comes in lots of shades and can be a source of silent delight as well as guffaws (actually, I don't think I've ever guffawed: that can only really be done by people with hair growing out of my ear holes).
Anyway, the book which I think has made me laugh the most is The Book Of Heroic Failures by Stephen Pile. My copy has cartoons by Bill Tidy.
It claims to be the Official Handbook of the Not Terribly Good Club of Great Britain.
The book is full, as one might expect, of people failing on a heroic scale. Like this example of someone picking the worst possible moment to pursue his chosen career path:
Mr David Goodall of Barnsley...set off in January 1979 to do a bit of shoplifting. He had hardly entered Barnsley's British Home Stores when he was simultaneously seized by eight pairs of hands. The shop was holding a conference of store detectives at the time.
The book is chock-full of brilliant stories; but, with writing as with shoplifting, it's all in the timing, and at this Stephen Pile is no sort of failure at all.
Word To Use Today: failure. This word has been in English since the 1200s. It comes from the Old French faillir, from the Latin fallere, to disappoint, and is probabaly something to do with the Greek phēlos, which means deceitful.