Writing Exercises? A question for the hive mind

Anyone out there like to recommend a book on writing exercises that they've found to be helpful? I haven't used one in at least a dozen years, and have no idea what's out there, or what works and what doesn't.

I've been asked for suggestions and had to punt, so I'm hoping some of the writers on my friendlist will have ideas.

ETA: What I'm looking for are short writing exercises, something that could be done in about an hour, and thus suitable for writers' groups.