Programming Ruby – The Pragmatic Programmer’s Guide is a free book about Ruby linked on the Ruby documentation page. Quite old but for most properties of the language suitable. Of course, there are some more useful links on the Ruby documentation page like the Ruby Core Reference or the Ruby Standard Library Reference, which lead to the Ruby documentation project.
O’Reilly has two free german books available on the internet. The first is O’Reilly Praxiswissen Ruby about the language Ruby and the second is O’Reilly Praxiswissen Ruby On Rails about Ruby on Rails. The last is mainly about Ruby on Rails 2 and for starters highly likely not interesting since Ruby on Rails 3 was released.
Ruby on Rails Tutorial is a great book about Ruby on Rails 3, which is free available on the internet. It is also purchasable as printed version or as pdf version on the site. In the book you will create a twitter simliar page by a test driven approach and with source code management by git. A short introduction to Ruby is certainly present in the book. On the side of Ruby on Rails can be found also some useful stuff. Two of them are the Ruby on Rails documentation and the Ruby on Rails tutorials.
Lastly some more links: