The target audience include: Computer Science Students, Programmers, Interested Amateurs, and Research Scientists in the fields of Biologically Inspired Computation, Machine Learning and Artificial Intelligence.
Really, the book is for a the curious student, early career researcher or programmer.
Some books in this field have covers have ugly abstract images from the field of evolutionary art. Even worse, some have dry depictions of math problems. I'm generally not a fan of any of these approaches (although, I'm not adverse to abstract imagery). This field of research is exciting and fun to study, the cover must suggest that to the target audience.
A possibly interesting creative direction to explore would be the nature of the book itself: the aggregation of a host of disparate algorithms under one umbrella - like an encyclopedia. A standardization effort that, like the algorithms themselves, take the many small parts together and provides (hopefully) something more than the sum.
A table of contents for the book is attached.
For motivation, the Wikipedia entries of Particle Swarm Optimization, Ant Colony Optimization, and the Genetic Algorithm, may provoke the creative juices. The art on http://www.complexification.net/, although unrelated, evokes a familiar visual feel to watching visualizations of these algorithms.