This project was intended to be a one-to-one clone of Mary Rose Cook's Little Lisp interpreter. Written in Ruby instead of Javascript though.
The original blog post with detailed explanations could be found here: http://maryrosecook.com/post/little-lisp-interpreter and the corresponding Ruby code is in the little_lisp_original.rb file.
However the main code diverged a little and could be found in little_lisp.rb file. Tests are written in RSpec and could be run from the command line by simply invoking spec command in the project's folder.