"This is a very very hard question, so many artists/illustrators inspires me. There is one way I can answer your question, the artist that made me realize that I wanted to illustrate for books was Mary Grandpre. Her beautiful cover illustrations were what attracted me to the harry potter books. They felt magical to my eyes."
So here is this week illustrator: Mary Grandpre:
Ah so magical!!!
I love how she describes her style, she calls it "Soft Geometry". It's a great way to describe it! there is indeed some softness to her illustrations along with beautiful long strokes in her shapes. She works mostly with soft pastels. I love her color palettes and how she composes each illustration (you can see it very well on her Harry Potter cover illustrations.)
Now here is a wonderful video of her drawing Harry: http://vimeo.com/7163263
Here is an interesting article on her: http://www.time.com/time/arts/article/0,8599,1810485,00.html
Here is her website: http://marygrandpre.com/
PS: Of all her Harry book covers my favorite one is "Harry Potter and the Goblet of Fire". What's yours?