My favorite book of all time, by far, without contest, is Philip Pullman's "The Amber Spyglass". I love everything about it. I love this book so much that I was convinced that I'd never illustrate it because there's just too many things I'd want to do. (If you haven't read the book and don't want things spoiled, skip the next paragraph.)
However, a scene from from the book is probably a good place to start. If you've read the book you'd probably recognize the scene as the death of Metatron. I chose this scene for a number of reasons. Firstly, it is one of the most identifiable scenes. Secondly, the scene was incredibly powerful. Thirdly, the complexity of Mrs. Coulter's character astounds me and this scene confuses me the most with her character, which is what I believe she is meant to do, confuse you.
I've included details of this piece for once.