Foundations of Programming by Karl Seguin

Going through the PDF document Karl Seguin was kind enough to share with all of us, could not not to agree with the written - quoting:

The real reason we’re spending a whole chapter on decoupling (which layering is a high-level implementation of) is because it’s a key ingredient in writing testable code. It wasn’t until I started unit testing that I realized how tangled and fragile my code was. I quickly became frustrated because method X relied on a functional class Y which needed a database up and running. In order to avoid the headaches I went through, we’ll first cover coupling and then look at unit testing in the next chapter.

Recommended reading.