What books / articles (large) are there on the topic of developing a site engine (CMS)?

Question

CMS

There are many small articles on the net, some of them are written, of course, very well, but due to their volume they do not provide very detailed information, I would like to delve into all the details of this topic and try to implement something.
Basically, I need this in order to later participate in the development of a "big" engine like Joomla, Drupal or WordPress, that is, I want to write my own, small one, get experience, a more or less detailed idea, and then try to delve into a more serious project and something do something for him (naturally, there is knowledge and experience in PHP + MySQL development)
Thank you for your attention to the issue.

Answer 1

CMS is the end product. At the heart of the CMS should be a core, or framework. You can take a finished one, you can write another bike.
I recommend you take Silex and Doctrine DBAL (not ORM) + PHP-DI and play around.
You are not yet destined to write products like Drupal / Joomla / Wordpress, you have too little experience for this, and therefore, in the process of development and design, you will not be able to predict even half of all user cases. And so the output will be another piece of guano.
Better concentrate on the basics.

Answer 2

Drupal is a good engine. Learn how it works under the hood. Open issues on Github. Solve them, send a pull request.