There is a whole list of e-books about ruby on rubyandrails.info Show archive.org snapshot ! Some of them are even free and you can filter it. How nice is that?!
Some honorable mentions:
Ruby Science
The book consists of a collection of code smells and anti-patterns and how to refactor them. Very important for everyone working in a team on a larger codebase!
Mastering Ruby Strings and Encodings
Programming is just a very advanced way to concatenate strings... It's always good to understand the underlying concepts in depth. This book is a smashing source of information and written in a way that is enjoyable to read. Every ruby dev should read this!
Ruby Regexp
All about using Regexes in Ruby. Probably good as reference book. Some concepts could be explained in more detail, but if you already know regex and just struggle to use it in Ruby, the book will be great.
Rubyfu
More a reference book for people who are into hacking or infosec and prefer Ruby.
