adding racket

free-programming-books.md

@@ -51,6 +51,7 @@
 * [PostgreSQL](#postgresql)
 * [Python](#python)
 * [R](#r)
+* [Racket](#racket)
 * [Ruby](#ruby)
 * [Ruby on Rails](#ruby-on-rails)
 * [Rust](#rust)
@@ -544,6 +545,10 @@
 * [Computational Statistics, Jeremy Penzer](http://stats.lse.ac.uk/penzer/ST419materials/)
 
 
+###Racket
+* [Programming Languages: Application and Interpretation](http://cs.brown.edu/courses/cs173/2012/book/index.html) 
+
+
 ###Ruby
 
 * [Learn Ruby the hard way](http://ruby.learncodethehardway.org/book/)