That's the Core reference guide. You might want the book "Programming Ruby"[1], which is linked on the documentation landing page[2]. Alternatively, for a quick overview, I recommend Learn X in Y Minutes[3].

[1]: http://ruby-doc.com/docs/ProgrammingRuby/

[2]: https://www.ruby-lang.org/en/documentation/

[3]: https://learnxinyminutes.com/docs/ruby/