Cleanup other books.

README.md

 ### Other Books
 *Books related to general computing and web development.*
 
-* [Elasticsearch: The Definitive Guide](https://www.elastic.co/guide/index.html) - A guide to working with Elasticsearch by Clinton Gormley and Zachary Tong.
 * [Eloquent JavaScript](https://eloquentjavascript.net/) - A book about JavaScript programming by Marijn Haverbeke.
-* [Head First Design Patterns](https://www.oreilly.com/library/view/head-first-design/0596007124/) - A book that expains software design patterns.
 * [Pro Git](https://git-scm.com/book/en/v2) - A book about Git by Scott Chacon and Ben Straub.
 * [The Linux Command Line](http://linuxcommand.org/tlcl.php) - A book about the Linux command line by William Shotts.
 * [The Tangled Web — Securing Web Applications](https://www.amazon.com/Tangled-Web-Securing-Modern-Applications/dp/1593273886) - A book about securing web applications by Michal Zalewski.
-* [Understanding Computation](https://computationbook.com) - A book about computation theory by Tom Stuart.
-* [Vagrant Cookbook](https://leanpub.com/vagrantcookbook) - A book about creating Vagrant environments by Erika Heidi.
 
 ### PHP Videos
 *Fantastic PHP-related videos.*