Actually, I found a book in my university's on going library book sale written in the forties, entitled
What's Funny and Why. by Milton Wright. It is interesting in a interesting sort of way, because 1) it was written in the forties when the humor industry was in the movies, and running off the last of the vaudville steam. 2) It was written before the civil rights politically correct movements 3) It was written forty years before I was born and thus while many of the jokes I get, there are a lot I really don't. 4) it is mostly verbal humor. 5) it often gets quite pretentious and smug.
Personally, I am inclined to agree with Lou Jacobs and Jamz, though it is unhelpful until you try things on people. But hey what are sisters for?
Also there was a really neat Infinite Mind (a hour long Radio show on NPR) show about laughter... ah here it is... there is a interview with the hilarious Eddy Izzard on it...
http://www.lcmedia.com/mind471.htm