It's a great idea to have seasonal books, that only come out during certain holidays like Hallowe'en, Hannukah, Easter, or Christmas.
Most of your child's favourite authors will likely have at least one Hallowe'en book. The Berenstain Bears, Robert Munsch, Clifford, Froggie, Angelina Ballerina, Arthur, The Rugrats, and magazines like Chirp, to name a few.
You can buy them new, but I like to start looking at this time through the bins at Goodwill and at garage sales and church bazaars. You can usually get them very inexpensively and they're usually not overly abused, since they're seasonal.
Here's a list from Amazon, which someone compiled of their favourite Hallowe'en books for kids aged 4 to 8. And here's another collection of Hallowe'en books featuring kids' favourite characters like Clifford and Arthur.
I find that even very young books, if they're about Hallowe'en, are fun for kids no matter what their age. When you bring them out of storage, there's a brief novelty to them that makes kids want to pick them up. They're only going to read them quickly once or twice anyway, and then back up into the attic they go - until next year.
Books like Winnie the Witch are not specifically for Hallowe'en, but they work just fine. And for grandparents - they make a great gift to send to your grandchild for Hallowe'en.
So... I went to Yellowknife and I missed Word on the Street last weekend. Argh! Oh well, I still have last year's Word on the Street pictures to reminisce with. Plus, Yellowknife was amazing - well worth it.