At our children's library, they always have a seasonal display of books, and the last time we visited, one of my children grabbed Pumpkin Pumpkin by Jeanne Titherington. This is an older book - published in 1986 - but we have loved it. The text is very simple and aimed at a young audience. The illustrations are beautiful and open the age range of this picture book to an older audience than the text would alone. The story and illustrations take you on a journey from a pumpkin seed to a jack-o-lantern in a very straightforward and unassuming way.
On of our favorite Halloween books is Sheep Trick or Treat by Nancy Shaw. This book was given to my daughter by her grandmother as a Halloween gift a few years ago and has been well loved. My four-year-old son especially thinks this book is hilarious. It is a simple rhyming scheme with fun illustrations that tell the story of the barn yard animals dressing up and trick-or-treating for Halloween. It is fun to see the costumes and what the different animals give out as "treats." This is a great Halloween book for a younger audience.