added a JS DOM book from DigitalOcean (#6099)

* Update free-programming-books-langs.md

* rearranged it in alphabetical order

* Update free-programming-books-langs.md

books/free-programming-books-langs.md

@@ -1109,6 +1109,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Thinking in JavaScript](https://www.amazon.com/Thinking-JavaScript-Aravind-Shenoy-ebook/dp/B00JUI6LUQ) - Aravind Shenoy (Kindle)
 * [Understanding ECMAScript 6](https://leanpub.com/understandinges6/read) - Nicholas C. Zakas (HTML)
 * [Understanding JavaScript OOP](http://robotlolita.me/2011/10/09/understanding-javascript-oop.html) - Quil (HTML)
+* [Understanding the DOM — Document Object Model](https://www.digitalocean.com/community/books/understanding-the-dom-document-object-model-ebook) - Tania Rascia (PDF, EPUB)
 * [You Don't Know JS](https://github.com/getify/You-Dont-Know-JS) - Kyle Simpson (HTML, PDF, EPUB, MOBI)