For younger children (and older children as well, as I personally enjoy this book myself), I really recommend The Witch Who Was Afraid of Witches by Alice Low. This is a really good friendship and confidence book, with a lot of fun Halloween images. A great book I discovered this year is Vampire Boy's Goodnight by Lisa Brown. This book has some especially wonderful references to classic monsters and the legend of Dracula. I would also recommend Moonlight: the Halloween Cat by Cynthia Rylant, especially for its wonderful, calming tone.
For older readers I would like to recommend (again) the Scary Stories collection by Alvin Schwartz and any of Daniel Cohen's stories, especially The Headless Roommate and Other Tales of Terror. There are a few others I think are really good reads, including Scary Stories, illustrated by Barry Moser, and Tales for the Midnight Hour by J. B. Stamper. There is a story in Tales for the Midnight Hour that I find particularly chilling titled The Attic. Definitely do not read it alone at night when everyone else is asleep, especially if you have an attic.
Happy Halloween!