Mr. Neighborly's Humble Little Ruby Book
The online version of this very neat book about the Ruby language is free, a paper version can be ordered for $9.95 US.
This is a single archived entry from Stefan Tilkov’s blog. For more up-to-date content, check out my author page at INNOQ, which has more information about me and also contains a list of published talks, podcasts, and articles. Or you can check out the full archive.