Design and Prototyping for Drupal by Dani Nordin
This will give an overview of how Drupal outputs content, and how to design around that visually, as well as an overview of turning the design into a theme. Less of a comprehensive, these are all the hooks you'll ever have to deal with volume, and more of a this is how to understand how Drupal works so you know *what* you're applying the visuals to thing.