http://oreilly.com/catalog/9781449302054 says: "With this digital Early Release edition of MacRuby: The Definitive Guide, you get the entire book in its earliest form -- the author's raw and unedited content -- so you can take advantage of this content long before the book's official release. You'll also receive updates when significant changes are made, as well as the final ebook version. All of this for a fraction of the price of the printed book."
However, the only buying option is to pre-order the print book.
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