In the words of Neil Gaiman, creator of All Hallow's Read (he's so silly):
Basically, you give someone a scary book for Halloween. Appropriately scary, of course. It can be a kid, a grown up, anyone you know (or don't know. Even strangers like to read. And who doesn't want a free book?)
If you go the All Hallow's Read site, you will find lots of answers to FAQs and also cool extras, such as this book plate that you can print out and affix to the book you are giving away. Or print it onto card stock and make it a bookmark.
Here are a few of our family's favorite Halloween reads:
What are some scary Halloween books you'd recommend?