Added The Internals of PostgreSQL (#2256)

* Added The Internals of PostgreSQL

* Removed trailing slash

free-programming-books.md

@@ -1700,6 +1700,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [Postgres Official Documentation](http://www.postgresql.org/docs/)
 * [Postgres Succinctly](https://www.syncfusion.com/resources/techportal/ebooks/postgres) (PDF, Kindle) *(Just fill the fields with any values)*
 * [Practical PostgreSQL](http://www.faqs.org/docs/ppbook/book1.htm)
+* [The Internals of PostgreSQL for database administrators and system developers](http://www.interdb.jp/pg)
 
 
 ### PowerShell