In this part of the book you will find a few essential Drupal modules presented quite briefly, along with their most important functionalities. The purpose of the following chapters is to emphasize a few Drupal modules that are important to learn, rather than presenting all details about how to use them. To balance the somewhat scarce documentation of the modules, there are a number of practical examples, showing how and when different settings may be useful.
The suggested solutions to the exercises in this part are not as detailed as in part B, but the comments are a bit richer. The purpose of the exercises in part C is not to give step-by-step instruction in how to create certain functions, but to give an understanding of what you can accomplish with the modules and pointers on how to do it.
The concepts presented in this part require that you are very comfortable with the concepts in parts A and B. This could take several months even if you use Drupal every day.
This part of the book should be complemented with a chapter about the Relation module and probably one about Entity reference. I (Itangalo) also think Entity Construction Kit fits/will fit among essential site builder modules, but time will tell if I'm in a small minority or not.