Documentation

I had to learn everything from the web. Bulky books are a problem for me.

Almost to low level API docs
http://www.ruby-doc.org/core-1.8.7/

Good "Programming Ruby" book
http://www.ruby-doc.org/docs/ProgrammingRuby/

EventMachine

Very Intro PDF
http://everburning.com/wp-content/uploads/2009/02/eventmachine_presentation.pdf

Better Intro PDF
http://everburning.com/wp-content/uploads/2009/02/eventmachine_introduction_10.pdf

Very Basic Tutorial
http://20bits.com/articles/an-eventmachine-tutorial/

Low Level API
http://eventmachine.rubyforge.org/