Add new book.

README.md

 * [The Tangled Web — Securing Web Applications](http://www.amazon.ca/The-Tangled-Web-Securing-Applications/dp/1593273886) - A book about securing web applications by Michal Zalewski.
 * [Elasticsearch: The Definitive Guide](http://www.elasticsearch.org/guide/) - A guide to working with Elasticsearch by Clinton Gormley and Zachary Tong.
 * [Eloquent JavaScript](http://eloquentjavascript.net/) - A book about JavaScript programming by Marijn Haverbeke.
+* [Vagrant Cookbook](https://leanpub.com/vagrantcookbook) A book about creating Vagrant environments by Erika Heidi.
 
 ## PHP Videos
 *Fantastic PHP-related videos.*