In a recent daily perusal of DZone headlines, the link Ruby Best Practices free ebook caught my attention. This link takes one to the Ruby Best Practices - Book site, where the PDF can be downloaded free of charge or registration by start reading (free PDF). The same page also has links to the related blog, to a github repository of the book's source code, and to purchase a hardcopy of the book.
As is the case for most "Best Practices" books, this one is probably more attractive to those who are not completely new to Ruby. For the developer who is completely new to Ruby, the famous Programming Ruby: The Pragmatic Programmer's Guide is a good place to start. Although this freely available electronic edition can be a little dated, it is highly conceptual and covers basic syntax and the covered concepts and syntax are still highly relevant in Ruby.
Another resource for those new to Ruby is my article Accessorize Oracle Database with Ruby.