WordPress learning from the code in

My knowledge of WordPress is increasing quickly with little bits of time here and there, and I think I have finally found some functionality that will require me to develop a proper plugin, there doesn't seem to be a plugin that gives the same functionality of node references in Drupal (although that appears to be giving way to more generic entity references). Basically I want to add another post type and make a reference to it from normal posts. I have to trawl through the plugins some more but so far nothing does exactly what I want.

So far I have a single plugin for the site that is doing a few simple things that some people would put in the theme templates (but that didn't feel right). Also making some decisions about what to put in the meta description tag.
With a system like Drupal or WordPress is is common to start learning from the admin interface inwards, which although it seems natural is a mistake in my opinion if you have other programming experience. I found it much easier to start from empty templates and work inwards, using documentation, core code and other examples for inspiration. I am wishing that, and looking I have been introduced to Drupal that way.
Obviously still a lot to learn but once I have developed a proper plugin I will probably have ticked enough boxes to start work on my more major language learning project.