I'd recommend Impro by Keith Johnstone. He also has a sequel of sorts called Impro for Storytellers but having read both I'd say go with Impro.

It's actually a really interesting read in our modern age of status update storytelling. It deals directly with the art of "status play" as the key to being amusing. It's amazing how much of making people laugh and enjoy your company comes down to managing your display of status and being able to diminish it for other's pleasure.

I have the curse/gift of growing up in a Maritime family with deep Scottish and Irish roots. Talk about a group that values the art of diminished status.