The Pragmatic Programmers wrote a great book in Programming Ruby.
They just released a second edition which is up to date with Ruby 1.8.2, and has an extra 250 pages (Ruby keeps getting bigger and better!)
The second edition of the PickAxe book! A tutorial, language reference, and library reference all in one book. Chapters on web programming, unit testing, extending Ruby, and packaging your code. New in this edition: six new tutorial chapters, documentation on all standard libraries, updated reference for built-in classes and modules. More than 250 pages of new or significantly updated material. Covers the latest Ruby (1.8).