This book is pretty well known now, but I thought I'd mention it because I love it so completely. This was author/illustrator/animator Mo Willems debut book. It came out only a few short years ago (2003) but now Mo and his pigeons and knuffle bunnies seem to be everywhere (which is fine with me).
I have to admit, at first glance, it appears to be relatively unsophisticated, but deeper examination reveals its complexity. Illustrated in an extremely straightforward fashion with flat colors and a black crayon, the artistic style is not beautiful or breathtaking but it expertly conveys the story and character with just a few lines. The immaturity of the pigeon is well suited to this child-like style of illustration. Willems uses every inch of space to tell his story, including endpapers, title page and even the covers. He also employs subtle changes in background color and excellent use of white space. Through these techniques and a hysterical and simple text, the conflict slowly and expertly builds to a very clear and humorous climax. The flawless unity of illustration and story make this book so compelling... and makes my three year old laugh every single time he reads it (which is a LOT).