I liked the Pragmatic Programmers Ruby Guide - Pickaxe.  You can find
an older version online for free or the latest edition (works with 1.8)
at any bookstore.
http://www.rubycentral.com/book/

Why's Poignant Guide is another good one.
http://poignantguide.net/ruby/

I would recommend using those books in parallel with Ruby Quizzes.

_Steve