Added Mercurial The Definitive Guide 2nd ed

free-programming-books.md

@@ -1508,6 +1508,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 
 * [HGInit - Mercurial Tutorial by Joel Spolsky](http://hginit.com)
 * [Mercurial: The Definitive Guide](http://hgbook.red-bean.com)
+* [Mercurial: The Definitive Guide 2nd edition](https://book.mercurial-scm.org/) *(in progress)*
 
 
 ### Mercury