Halloween by Laura Marsh is a Level One National Geographic Reader, which is "just right for kids who are beginning to read on their own", according to the back cover of the picture book.
Basically, this is a National Geographic Kids perspective on Halloween; if your child reads that magazine (and I recommend it!), they should enjoy this book.
It's a very simple picture book for young readers, and it presents a simplified version of the history of Halloween, El Dia de Los Muertos, and pumpkin/jack-'o-lantern traditions, interspersed with rhymes, jokes, and ideas for decorations and costumes. It's only thirty-two pages long, so it should be a quick read for a young child getting into the holiday spirit.