I believe that any Rails developer which think that Java/JVM is enemy camp should read it. The tools which are highlighted in this book are very interesting and provide for Ruby/Rails developers broader space.
As for Java developers it's a good start point for integration Rails in corporate Java environment. The reader get all required info to get started.
Here is a list of tools:
- Warbler (https://github.com/jruby/warbler) - Warbler provides a minimal, flexible, Ruby-like way to bundle up all of your application files for deployment to a Java environment
- Trinidad (https://github.com/trinidad/trinidad) - Trinidad allows you to run Rails or Rack applications within an embedded Apache Tomcat container.
- Kirk (https://github.com/strobecorp/kirk) - Kirk is a wrapper around Jetty that hides all of the insanity and wraps your Rack application in a loving embrace
- Torquebox (http://torquebox.org) - TorqueBox is a new kind of Ruby application platform that supports popular technologies such as Ruby on Rails and Sinatra, while extending the footprint of Ruby applications to include built-in support for services such as messaging, scheduling, caching, and daemons.
- jetpack (https://github.com/square/jetpack), it wasn't mentioned in book. It packages your JRuby webapp for Jetty.