Security can be such a nebulous topic, bereft of practical examples of how "bad things happen". This book stands out because it walks you through exploiting and fixing a real Ruby on Rails application! I have found these examples invaluable in explaining away the "haze" of security to my colleagues, further grounding their knowledge in practice instead of theory.
Additionally, this book serves as a great "cookbook" for implementing security programming paradigms. Ever wonder how to do Symmetric Key Encryption with OpenSSL in Ruby? See Chapter 8. Unit Testing Authentication? Chapter 4. Wrestling with Kerberos? Chapter 10.
The writing is concise and object, with a strong focus on examples and getting things done. A refreshing alternative to the "Alice, Bob, Carol" analogies you get in the more theory heavy books. If I want to know how Diffie-Hellman works, I'll go look it up - this book shows me how to Get It Done!