Merge branch 'pr/2080'

free-programming-books.md

@@ -1509,6 +1509,7 @@ 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)* ([source repository](https://bitbucket.org/hgbook/hgbook))
 
 
 ### Mercury