I recently worked on the illustrations for the second edition of the free e-book "Don't Just Roll The Dice", written by Neil Davidson and published by Efendi books, which you can download here. The illustrations were very fun to do, and I also got to work with traditional watercolours ("proper" paint!) which I found much harder than digital watercolours due to a lack of the Edit>Undo function. The illustrations were all whimsical takes on the chapter titles, so "Pricing pitfalls" became a man falling down through a cloud of money, for example.
The book is about how to price software, and it a really concise, clear read for all you computer scientists. I'd recommend you have a look!