"The Ruby Programming Language" by Flanagan and Matsumoto may not have that as it's main goal, but gives a very good overview on why features are how they are a

"The Ruby Programming Language" by Flanagan and Matsumoto may not have that as it's main goal, but gives a very good overview on why features are how they are and even gives some advice from the creator of the language himself and some commentary on them (e.g. flip-flops are considered an implementation oddity kept around for backwards compat). http://www.amazon.com/Ruby-Programming-Language-David-Flanagan/dp/0596516177 http://www.amazon.com/Ruby-Programming-Language-David-Flanag... I think it's the most relevant Ruby book, because it documents the basic thinking and mental approach of the language, which is actually very stable.