The Well-Grounded Rubyist is one of my all-time favorite programming books. I normally loathe books focused on learning the particulars of a programming language, instead preferring blogs and experimentation. Well-Grounded Rubyist is a huge exception, though. The book is aimed at experienced programmers new to Ruby, and thoughtfully walks you through the features of the language. I read the book twice cover to cover, and have given several copies to programmers looking to learn Ruby. The informal style of the book combined with the deep, comprehensive overview of the language is just magic and was key to landing my first Ruby gig.
