Some unique features of the Illustrator's Guidebook 1:
- Backgrounds: structure and elements
- Perspective: the various types and how they are used in illustrations
- Composition: the position and design the elements of your illustrations
- Hands and Feet tutorial
- Different artists' process for creating characters
Some unique features of the Character Designer:
- Drawing caricatures
- How to age characters (from baby to elderly person)
- Color theory and choosing good color schemes
- Realistic changes in shape in response to motion, force, squash and stretch
Some of the other content may overlap a bit. For example, both books discuss human anatomy as well as aspects of dynamic poses, such as flow and line of action.
However, these topics are taught by different artists who have their own unique style and approach to art. They often have a different way of explaining these concepts or focus on certain aspects that another artist doesn't.