I've worked on many embedded projects in my college, i also had a position of responsibility as coordinator of embedded systems and robotics club. I've designed many t-shirts and banners for our workshops, which involved illustrations which also gave idea about the final result of workshop, i can mail you samples which i've designed. I've posted 3 days because it's a book and it's cover must be designed very carefully. We can still continue to improve it in future if you get some better creative idea.