You could use them for a unique hostess gift for all the Halloween parties you attend this season. Give the bookplates themselves or spend a bit more time picking out some Halloween themed books that the hostess might like and add these bookplates to the inside front of the book - I know I would love a gift like this.
I couldn't decided if these bookplates from Manifesto Letterpress were "get your spook on" worthy or just plain cool. I'm so in love with them but do they really scream Halloween? In the end I decided that it just didn't matter because I dig them so much.