Here is a great link to a site entirely devoted to the funniest books ever written: Funny Books . org
My personal favourites on this list has to be Wilt by Tom Sharpe - the first in the Wilt series and particularly amusing because of its mid 70s setting, and of course the "trilogy in five parts" Hitchikers Guide to the Galaxy by Douglas Adams.
Missing from the list is Incompetent by Rob Grant, but the moderators have promised to add it shortly.
See the whole list here: Funny Books . org