Few friends and colleagues have asked me to provide them with a list of materials I used to study. The truth is, I did not study using any specific material, so
This is my recommendation
Do a lot of site building work focused mainly on using Drupal Core to its fullest and extending Drupal using contributed modules as much as possible instead of creating custom modules. Use Drupal recommended best practices.
Install Drupal core without any contributed module, make sure you know how much you can do with Drupal core alone. Think of the simply scenarios where you will need to use Views Modules, Date fields, when and how to create custom fields, custom content types, when and how to re-use fields, and when and to rename fields, taxonomy terms, vocabularies and content types with and without data. Study the permissions page and understand basic Drupal permissions and their implication especially administrative permissions. Create views and use views with exposed filters and etc. Make sure you know what happens when an administrator change the input filters of a long text field to one that is not accessible to the other content editors of the site.