Merge pull request #337 from chuanjin/master

Update free-programming-books.md

free-programming-books.md

@@ -522,6 +522,9 @@
 
 ###Erlang
 * [Learn You Some Erlang For Great Good](http://learnyousomeerlang.com/)
+* [Erlang Programming] (http://it-ebooks.info/book/325/)
+* [Programming Erlang] (http://www.e-reading.biz/bookreader.php/139986/Programming_Erlang.pdf)
+* [Concurrent Programming in ERLANG] (http://www.erlang.org/download/erlang-book-part1.pdf)
 
 
 ###Flex