Well when I buy a book I read it, just sort of makes sense to me.
Comedy writing secrets disects jokes and explains how they work and catogorises them.
I was a bit disconcerted by the example jokes being, well, not funny. It also gives you info on how to make some money with your creativity, but I wouldn't give up the day job.
I'd be interested in better books than this, but that's the only one I've seen.
I think what's a good idea, is just take more notice of your favourite comedies/comedians and try to work out what they are doing to be funny.
For instance, I like BlackAdder and Faulty towers, and guess what, I love sarcasm and irony. I also tend to use it alot.
My favourite comedian at the moment is Jimmy Carr. He's got an excellent video out at the moment and I'm nicking his jokes everyday (just like Jim Davison). Short punchy 2 liners, many excellent C&F material.
Use humour sparingly though, too much and I think you come across somewhat insecure, less is more.