This is an excellent book.
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Ruby Best Practices is a book I wish I had been able to read back in 2005 and 2006, when I started learning the language. Ruby gives you near-absolute freedom; with all those options, finding your feet can take some time. "Best practices" might carry a corporate overtone in some ears, but this book comes straight from the heart of the Ruby community (unlike some of O'Reilly's other Ruby books).