The beautiful thing about Sinatra is that it is ridiculously easy to pick up and go. With even a small amount of Ruby knowledge you're up and running in no time.
One of the things I love about Sinatra is that pretty much everything you need to know is in the README. Otherwise, understanding how Rack/your web server/your gems work is useful. From another angle, Paul Dix's Service-Oriented Design with Ruby and Rails has a lot of examples of using Sinatra in a variety of applications.
