Worst. Advice. Ever.
Once you’ve figured that all out then upgrade and expect to spend the same amount of time learning all the new stuff. You don’t save any time jumping to Rails 3+.
This just isn't true.
I strongly urge the author to change this suggestions because it's a dangerous one. You don't want newcomers learning an older version of Rails and then struggling to upgrade.
There are a lot of resources to help you out. The API Docs[1], Official Rails Guides[2] are excellent and Railscasts[3] are also awesome.
[1] Rails API Docs http://api.rubyonrails.org [2] Rails Guides http://guides.rubyonrails.org [3] Railscasts http://railscasts.com
As a front-end developer with little back-end experience, I was able to get the basics of Rails with just a few hours working through Hartl's tutorial.