Konstantin Haase & Alan Harris 
Sinatra: Up and Running [PDF ebook] 
Ruby for the Web, Simply

Take advantage of Sinatra, the Ruby-based web application library and domain-specific language used by Heroku, Git Hub, Apple, Engine Yard, and other prominent organizations. With this concise book, you will quickly gain working knowledge of Sinatra and its minimalist approach to building both standalone and modular web applications.Sinatra serves as a lightweight wrapper around Rack middleware, with syntax that maps closely to functions exposed by HTTP verbs, which makes it ideal for web services and APIs. If you have experience building applications with Ruby, youll quickly learn language fundamentals and see under-the-hood techniques, with the help of several practical examples. Then youll get hands-on experience with Sinatra by building your own blog engine.Learn Sinatras core concepts, and get started by building a simple application Create views, manage sessions, and work with Sinatra route definitions Become familiar with the languages internals, and take a closer look at Rack Use different subclass methods for building flexible and robust architectures Put Sinatra to work: build a blog that takes advantage of service hooks provided by the Git Hub API