Getting upto speed with Rack - links, articles etc

I'm sure everyone knows of Rack but perhaps some may not know exactly what rack does. So if anyone has any links to any interesting background reading please post them here.

Heres a few

I was surprised to see that About.com has a nice short one page explanation here
http://ruby.about.com/od/rack/a/What-Is-Rack.htm

Ryan Bates also has free introductions here
http://railscasts.com/episodes/151-rack-middleware
http://railscasts.com/episodes/222-rack-in-rails-3

and a pro screencast detailing how to make your own app with rack here
http://railscasts.com/episodes/317-rack-app-from-scratch

The Rack author has an introductory blog post
http://chneukirchen.org/blog/archive/2007/02/introducing-rack.html

and here's another one here
http://m.onkey.org/ruby-on-rack-1-hello-rack

One last one before I get to sleep
This author shows how they used rack to solve a real world problem - however it is 3 years old but should show how rack can be used.

http://thewebfellas.com/blog/2008/12/22/flash-uploaders-rails-cookie-based-sessions-and-csrf-rack-middleware-to-the-rescue

[hectorsq]

This is a free intro to Rack ebook by Satish Talim from rubylearning.org

http://gallery.mailchimp.com/e49655551a5bb47498310c7de/files/RackIntro.pdf

[stuartellis]

Here's a Rails guide that discusses how Rack is used in Rails: http://guides.rubyonrails.org/rails_on_rack.html

[stuartellis]

I found this a nice simple introduction with working code:

http://amberbit.com/blog/introduction-to-rack-middleware

[stuartellis]

The Toto blog engine is a one-file Rack application:

https://github.com/cloudhead/toto/blob/master/lib/toto.rb

[samnang]

I found and really enjoy remi's screencasts:

http://remi.org/2009/02/19/rack-basics
http://remi.org/2009/02/24/rack-part-2
http://remi.org/2009/02/28/rack-part-3-middleware