Would be good to be able to find a super quick ruby course for regular PSEs just give them some idea of what they are looking at. Senior PSEs could then do a more advanced course.
For now, here's a few links and books:
As an easy way to get into Ruby for people who have close to no programming experience, I always recommend X for Kids series of books. In Ruby's case, I'd recommend Ruby Wizardry
The Ruby Website itself has a section dedicated to exploring Ruby from the viewpoints of somebody who already knows other languages: https://www.ruby-lang.org/en/documentation/ruby-from-other-languages/
This can be very useful to people who already know a language or two, but not ruby.
For extremely advanced use, i used to recommend Ruby Under a Microscope
It has a chapter on "Metaprogramming", and one on "JRuby: Ruby on the JVM" both of which are very relevant to Ruby in Puppet, so i wouldn't dismiss it just based on its age (2013)