It's no secret that ruby is the current language of choice among trendy developers, most of them seduced by the Ruby on Rails web framework. To be proficient with any framework, one must master it's underlying language, and this PDF book perfectly suits this need.
Author: Huw Collingbourne
Valoration: 6 / 10
Book website: http://www.sapphiresteel.com/The-Book-Of-Ruby
Download link: http://www.sapphiresteel.com/IMG/zip/book-of-ruby.zip
Having the book's author an MA in English from the University of Cambridge is an assurance of smooth reading. And I can say he doesn't deceives: it's hard to find a (free) book made by a software developer such well written. Another topic is its design...
If you are looking for fancy graphics & nice design like in Book 1: 12 Things to Shorten Your Lead Time, you won't find them here: the book goes straight to the point, explaining all essential idioms of ruby from the basics (data types, flow control) to more advanced topics (regular expressions, threading, dynamic programming). Worth to mention is a full chapter devoted to Rails, covering many parts of it and giving a good introduction.
The greatest defect of this book is the source code samples' font choice: let's face it, Comic Sans is not suited at all for this task! I really missed here a monospaced (fixed-width) font like Courier New. Also some type of remarking of the code snippets would have been welcome (e.g. surrounding them in a gray box), as this helps to keep focused and better understand them. Another annoyance is the lack of a table of contents in the book: you must search back and forth thorough it to find a specific chapter. At least each odd page has and indication of which chapter it belongs to and has an index at the end of the book...
For all the explained, I've found this book more appropriate for learning ruby than as a reference book. Although the relatively low valoration I must say it is an enjoyable reading and a very complete book.
As a remarkable bonus I should mention that the zip includes the source code of all the chapters in the book. You hardly can get more for less :D