So far we've covered two books in the official* Oz canon with Halloween-ish elements. But are there any books about Halloween itself? Well of course there are! Masquerade in Oz by Bill Campbell and Irwin Terry is about a Halloween party in the Emerald City where the Oz celebrities dress up like each other! Of course that doesn't make for much of a book, so of course things have to go wrong somehow, and that happens when Scraps accidentally casts a spell that makes everyone think they are the person they are dressed as. Yeah, that doesn't work out very well at all. If you're looking for a good, fun Oz romp that celebrates the holiday, you can get your hands on this book at the link above or right here.
* Well, as official as Oz canon gets, as not everyone can agree as to what that actually means.