Interesting, but probably a bit out of date, as it is based on Ruby 2.0, and Ruby 4 has just been released. Also, we now have more concurrency primitives like Fibers and Ractors, as well as the Threads discussed in the article.
Pickaxe Book, Chapter 12 - but it is terser and aimed at someone with more experience. I think that the article is a good introduction to Threads, and once you understand how they work, then Fibers and Ractors start to become easier to understand.
