Add nodejs handbook (#6142)

books/free-programming-books-langs.md

@@ -1241,6 +1241,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Node.js Succinctly, Syncfusion](https://www.syncfusion.com/resources/techportal/ebooks/nodejs) (PDF, Kindle) (email address *requested*, not required)
 * [Practical Node.js: Building Real-World Scalable Web Apps](https://github.com/azat-co/practicalnode) - Azat Mardan
 * [The Node Beginner Book](http://nodebeginner.org)
+* [The Node.js Handbook](https://flaviocopes.com/page/node-handbook/) - Flavio Copes (HTML, PDF) *(email required for PDF)*
 
 
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