I usally just A9 for stuff if I run into a road block with a particlar class or something. The online docs seem to have some missing information, which presents a bit of a challenge.
Tom Standage: The Turk
The Life and Times of the Famous Eighteenth-Century Chess-Playing Machine
Dave Thomas: Programming Ruby: The Pragmatic Programmers' Guide, Second Edition
This book is affectionately known as the pick-axe in the Ruby community. It is *the* book to have.