It's been a long time coming, but The RSpec Book is finally shipping. And from what I've seen from the beta ebook, it's going to be quite good. If you've been waiting to order, now is the time!
RSpec is a behavior-driven development tool for Ruby. It's considered by many to be one of the core tools in the Ruby and Rails toolchest. Even for simple uses, RSpec shines. To put it short, if you don't know RSpec, you should even if you don't intend to use it yourself, because there's no escaping RSpec.