diff --git a/content/books/things-i-learnt/_index.md b/content/books/things-i-learnt/_index.md
index 18fb4b8..12e3399 100644
--- a/content/books/things-i-learnt/_index.md
+++ b/content/books/things-i-learnt/_index.md
@@ -36,6 +36,7 @@ template = "section-contentless.html"
 	* Project Organization
 		* [Organize Your Code by Data/Type, Not Functionality](project-organization)
 		* [Create Libraries](libraries)
+		* [The Config File Is Friend](config-file)
 	* Writing code
 		* [Be Ready To Throw Your Code Away](throw-away)
 		* [Future Thinking Is Future Trashing](future-trashing)
diff --git a/content/books/things-i-learnt/config-file/index.md b/content/books/things-i-learnt/config-file/index.md
new file mode 100644
index 0000000..3723326
--- /dev/null
+++ b/content/books/things-i-learnt/config-file/index.md
@@ -0,0 +1,37 @@
++++
+title = "Things I Learnt The Hard Way - The Config File Is Friend"
+date = 2019-07-15
+
+[taxonomies]
+tags = ["en-au", "books", "things i learnt", "configuration", "config file"]
++++
+
+Do not ignore the power of configuration files.
+
+<!-- more -->
+
+Imagine you wrote a function that you have to pass a value for it to start
+processing (say, a twitter user account id). But then you have to do that with
+two values and you just call the function again with the other value.
+
+It makes more sense to use a config file and just run the application twice
+with two different config files 'cause, this way, you have a single, small,
+testable application instead of two, or a very complex application that does a
+lot of stuff.
+
+We can even jump into the idea of [creating
+libraries](/books/things-i-learnt/libraries) and say that, instead of
+splitting your e-commerce application into smaller parts and making a big one
+by grouping these smaller parts, you could simply have one e-commerce
+application and, for each of your clients, you would have a different
+configuration file, pointing to different assets. This way, even the assets
+may reside in the same repository in the same branch, 'cause all that
+identifies which assets should be used are defined in the configuration file.
+
+"But which one should I use?" you may ask. Well, "it depends". It may make
+sense to have one single application with different configuration files if
+most of its can be used all the time. If the intersection of used things is
+very small, it may make more sense to split into different libraries and just
+"pick and chose" what to use.
+
+{{ chapters(prev_chapter_link="/books/things-i-learnt/libraries", prev_chapter_title="Create Libraries", next_chapter_link="/books/things-i-learnt/throw-away", next_chapter_title="Be Ready To Throw Your Code Away") }}
diff --git a/content/books/things-i-learnt/libraries/index.md b/content/books/things-i-learnt/libraries/index.md
index ac3a563..2c4cd2a 100644
--- a/content/books/things-i-learnt/libraries/index.md
+++ b/content/books/things-i-learnt/libraries/index.md
@@ -45,4 +45,4 @@ your control, they are external to the project. So you may need to learn how
 to deal with this before creating the libraries. And, unfortunately, each
 language and build tool has its own way to manage this.
 
-{{ chapters(prev_chapter_link="/books/things-i-learnt/project-organization", prev_chapter_title="Organize Your Code by Data/Type, Not Functionality", next_chapter_link="/books/things-i-learnt/throw-away", next_chapter_title="Be Ready To Throw Your Code Away") }}
+{{ chapters(prev_chapter_link="/books/things-i-learnt/project-organization", prev_chapter_title="Organize Your Code by Data/Type, Not Functionality", next_chapter_link="/books/things-i-learnt/config-file", next_chapter_title="The Config File Is Friend") }}
diff --git a/content/books/things-i-learnt/throw-away/index.md b/content/books/things-i-learnt/throw-away/index.md
index 30e445c..7f4ace5 100644
--- a/content/books/things-i-learnt/throw-away/index.md
+++ b/content/books/things-i-learnt/throw-away/index.md
@@ -39,4 +39,4 @@ And not just code that solves the problem, but also the tests for that code.
 ... unless you focus mostly on [integration
 tests](/books/things-i-learnt/integration-tests).
 
-{{ chapters(prev_chapter_link="/books/things-i-learnt/libraries", prev_chapter_title="Create Libraries", next_chapter_link="/books/things-i-learnt/future-trashing", next_chapter_title="Future Thinking Is Future Trashing") }}
+{{ chapters(prev_chapter_link="/books/things-i-learnt/config-file", prev_chapter_title="The Config File Is Friend", next_chapter_link="/books/things-i-learnt/future-trashing", next_chapter_title="Future Thinking Is Future Trashing") }}