Definitions and Cover Examples

A sign is a stimulus pattern that has meaning. A frown signifies sadness.

An index is a sensory feature or signal pattern that correlates and implies a behaviorally relevant situation. Dark clouds mean rain.

A symbol is a thing that represents or stands for something else. A bald eagle is a symbol for the USA.

3 Examples of different styles of book covers

The above cover I think is fairly successful. It's simple and clean, yet witty. It uses the lightbulb as a symbol for what the book will be about. You think of an interrogation and you instantly see the clever connection of the lightbulb.

The above Lincoln book cover is successful in that it looks really neat how all the type forms his face. The problem I personally have with it is that there's a lot of text, and it's all caps, all close together. If this is a snapshot of how the book will be, I'm not as interested in picking it up to read it.

The cover above makes me want to cry. Not just because it's poorly designed, but also because of the bright obnoxious neon green. I have no clue what the little illustration thing has to do with the book, not to mention it partially covers the text and interferes with me reading it clearly. The text is also all the same size with only a slight color change in part of the title. This makes it hard to distinguish much of anything text-wise.