I think it was The Blogunteer that first pointed me to this book. It wasn't available on Amazon the last time I looked, but OnaTuesdayNight.com had this video that introduces the book.
Tammi DeVille wrote profiles of a bunch of different people that volunteer at different organizations. Some are once a month gigs, and some are 20 hours a week. The point is one that many volunteers will tell you:
There are plenty of opportunities out there. It doesn't take as much commitment as you think. Unless you want it to.
There were two organizations that I hadn't heard of before, that struck me as awesome:
First, there was the Pan-African Association, which mentors immigrant refugees from Africa. It seems there are lots and lots of refugees that emigrate to Chicago. Abby, the volunteer in the profile, helps with language skills, government forms and other "everyday life" things.
Another is The Infinite Family, a group that connects kids in southern Africa with mentors via webcam.
This isn't a book one would read like a novel, but it certainly gives one an idea of how much work there is to do and how much fun it can be to help.